<PAGE>   1
                                                                    EXHIBIT 4.12

                               DATED 15 APRIL 1999








                      CENTEX DEVELOPMENT COMPANY UK LIMITED




                                   INSTRUMENT




                                  constituting
                           Negotiable Loan Notes 2001

























                               LINKLATERS & PAINES
                                 One Silk Street
                                 London EC2Y 8HQ

                             TEL: (+44) 171 456 2000

                                  Ref: SGG/JLF



<PAGE>   2


                  "TRANSFER OFFICE" means Goldvale House, Church Street West,
                  Woking, GU21 1DJ or such other place within the United Kingdom
                  as the Directors may determine and notify each of the
                  Noteholders in writing where the Company is situate for the
                  time being.

         1.2      References herein to "THIS INSTRUMENT" or "THIS DEED" shall
                  include any Schedules hereto and references herein to Clauses,
                  Conditions, paragraphs, sub-paragraphs or Schedules are to
                  clauses, conditions, paragraphs, sub-paragraphs hereof or to
                  the schedules hereto.

         1.3      Subject as expressly defined any words and expressions defined
                  in the Companies Act 1985 and used herein shall have the same
                  meanings when used in this Instrument.

         1.4      References herein to any provision of any statute shall be
                  deemed also to refer to any statutory modification or
                  re-enactment thereof from time to time in force.

         1.5      Words used herein denoting persons shall include corporations,
                  the masculine gender shall include the feminine and the neuter
                  and the singular shall include the plural and vice versa.

         1.6      The headings herein are for convenience of reference only, do
                  not constitute a part of this instrument and shall not be
                  deemed to limit, extend or otherwise affect the meaning of any
                  of the provisions hereof.

         2        AMOUNT OF THE NOTES

                  The principal amount of the Notes constituted by this
                  Instrument is an amount equal to the sum of the Initial
                  Consideration and the Additional Consideration. The Notes
                  shall be issued fully-paid in denominations of (pound)1 in
                  nominal amount or integral multiples thereof and shall be
                  transferable in such amounts as provided in the Second
                  Schedule.

         3        STATUS OF THE NOTES

                  The Notes when issued shall rank pari passu equally and
                  rateably without discrimination or preference and as unsecured
                  obligations of the Company except for those obligations as may
                  be preferred by law. The Notes shall be known as "NEGOTIABLE
                  LOAN NOTES 2001".

         4        CONDITIONS OF ISSUE

                  The Conditions and provisions contained in the Schedules shall
                  have effect in the same manner as if such Conditions and
                  provisions were set out herein. The Notes shall be held
                  subject to and with the benefit of the Conditions and of the
                  provisions in the Schedules, all of which shall be binding on
                  the Company and the Noteholders and all persons claiming
                  through them respectively.

         5        COVENANTS BY THE COMPANY

                  The Company hereby covenants with the Noteholders and each of
                  them duly to perform and observe the obligations on its part
                  contained in this Instrument to the intent that this
                  Instrument shall enure for the benefit of all Noteholders each
                  of whom may sue for the performance or observance of the
                  provisions hereof so far as his holding of Notes is concerned.

         6        CERTIFICATES FOR NOTES

         6.1      Each Noteholder will, subject to the terms of this Instrument,
                  be entitled without charge to one Certificate for the
                  aggregate number of Notes registered in his name or, if so
                  requested by a Noteholder, 2 certificates each representing a
                  tranche of the Notes registered in his name and together
                  representing the aggregate number of Notes registered in his
                  name. Each Certificate shall bear a denoting number and shall
                  be executed by the Company. Every Certificate shall be

                                        2

<PAGE>   3




                  in the form or substantially in the form set out in the First
                  Schedule and shall have the Conditions endorsed thereon.

         6.2      The Company shall not be bound to register more than four
                  persons as the joint holders of any Notes and in the case of
                  Notes held jointly by several persons the Company shall not be
                  bound to issue more than one Certificate therefor. Delivery of
                  a Certificate to one of such persons shall be sufficient
                  delivery to all. When a Noteholder transfers or has redeemed
                  part only of his Notes, the old Certificate shall be cancelled
                  and a new Certificate for the balance of such Notes issued
                  without charge.

         7        REGISTER OF NOTES

         7.1      The Company shall at all times keep at the Transfer Office or
                  at its registered office a Register showing:

                  (a) the names and addresses of the holders for the time being
                  of the Notes and, in the case of joint holders, the names of
                  the joint holders and the address of the first named holder;

                  (b) the amount of the Notes held by each registered holder
                  and, in the case of joint holders, the amount of Notes held by
                  the joint holders taken together;

                  (c) the date on which the name of each individual registered
                  holder (including, in the case of joint holders, each joint
                  holder) is entered in respect of the Notes standing in his or
                  their name; and

                  (d) the number of each Certificate for the Notes issued and
                  the date of issue thereof.

                  Any change of name or address on the part of any Noteholder
                  shall forthwith be notified to the Company and as soon as
                  reasonably practicable (and in any event within 5 Business
                  Days) the Register shall be altered accordingly. The
                  Noteholders or any of them and any person (not being a person
                  to whom the Company may reasonably object) authorised in
                  writing by any Noteholder shall be at liberty, at all
                  reasonable times during office hours on any Business Day, to
                  inspect the Register.

         7.2      Except as required by law, the Company will recognise the
                  registered holder of any Notes as the absolute owner thereof
                  for all purposes and shall not (except as ordered by a court
                  of competent jurisdiction) be bound to take notice or see to
                  the execution of any trust, whether express, implied or
                  constructive, to which any Notes may be subject and the
                  receipt of the registered holder for the time being of any
                  Notes, or in the case of joint registered holders the receipt
                  of any of them, for the principal moneys payable in respect
                  thereof or for the interest from time to time accruing due in
                  respect thereof or for any other moneys payable in respect
                  thereof shall be a good discharge to the Company,
                  notwithstanding any notice it may have, whether express or
                  otherwise, of the right, title, interest or claim of any other
                  person to or in such Notes, interest or moneys. The Company
                  shall not be bound to enter any notice of any trust, whether
                  express, implied or constructive, on the Register in respect
                  of any Notes.

         7.3      Noteholders shall be entitled to receive a copy of this
                  Instrument without charge on application to the Company.

         8        REDEMPTION

                  By 11 am (London time) on 30 March 2001 (the "REDEMPTION
                  DATE"), the Company will pay to each of the Noteholders the
                  principal amount of the Notes held by the relevant Noteholder
                  on that date.


                                        3

<PAGE>   4




         9        GOVERNING LAW

                  This Instrument and the Notes shall be governed by, and
                  construed and enforced in accordance with English law without
                  regard to the conflict of law rules thereof. The Company
                  irrevocably agrees that the Courts of England are to have
                  exclusive jurisdiction to settle any disputes which may arise
                  out of or in connection with this Instrument and the Notes.

                  IN WITNESS whereof this Instrument has been executed and
                  delivered as a deed on the date first stated above.

         EXECUTED as a DEED by
         CENTEX DEVELOPMENT COMPANY     )
         UK LIMITED                     )
         acting by                      )


         Director              /s/ Stewart A. Baseley

         Director/Secretary    /s/ Paul M. Bak







                                        4

<PAGE>   5

                               THE FIRST SCHEDULE

                              FORM OF CERTIFICATE
<TABLE>
<CAPTION>

         CERTIFICATE NO.               ISSUE DATE             AMOUNT

<S>                                    <C>                    <C>
                                                              The aggregate of (i) (pound) and
                                                              (ii) amount of the Additional
                                                              Consideration referred to in the
                                                              Instrument.
</TABLE>

                      CENTEX DEVELOPMENT COMPANY UK LIMITED

         (Incorporated with limited liability under the laws of England
                         with registered number 3720116)

                           NEGOTIABLE LOAN NOTES 2001

         THIS IS TO CERTIFY THAT the undermentioned is/are the registered
         holder(s) of the amount set out below of the Negotiable Loan Notes 2001
         constituted by an instrument entered into by the Company on l5 April
         1999 (the "INSTRUMENT") and issued with the benefit of and subject to
         the provisions contained in the Instrument. Where the context so
         admits, words and expressions defined in the Instrument shall bear the
         same meanings in the Conditions endorsed hereon.

         This Certificate is evidence of entitlement only. Title to the Notes
         passes only on due registration on the Register and any payment due on
         the Notes will be made only to the duly registered holder.

         NAMES OF HOLDER(S)                      AMOUNT OF NOTES






         SIGNED AND DELIVERED AS A DEED
         by CENTEX DEVELOPMENT COMPANY UK LIMITED
         ACTING BY:

         Director /s/ [Illegible]              Director/Secretary

         DATED:       APRIL 1999

         NOTES:

         1        The Notes are repayable in accordance with the Conditions
                  endorsed hereon.

         2        This Certificate must be surrendered before any transfer,
                  whether of the whole or any part of the Notes comprised in it,
                  can be registered or any new Certificate issued in exchange.

         3        The Notes are transferable in accordance with the conditions
                  endorsed hereon and this Certificate must be lodged together
                  with the instrument of transfer (which must be signed by the
                  transferor or by a person authorised to sign on behalf of the
                  transferor) at the Transfer Office.

         4        A copy of the Instrument is available for inspection at the
                  Transfer Office at all reasonable times during office hours on
                  any Business Day. Noteholders shall be entitled to receive a
                  copy of the Instrument and its Schedules without charge on
                  application to the Company.

         5        No interest is payable on the Notes.


                                        5

<PAGE>   6





                               THE SECOND SCHEDULE

                                 THE CONDITIONS

         1        REPAYMENT, PURCHASE AND REDEMPTION

                  Unless previously repaid, redeemed or purchased by the Company
                  the amount of the Notes held by each Noteholder shall be
                  repaid by 11 am (London time) on 30 March 2001. Payment of the
                  amount of the Notes shall be in the manner specified in
                  Condition 2.

         2        PAYMENT

                  Payment under the terms of this Instrument shall be effected
                  by crediting on the relevant payment date the account
                  specified by the relevant Noteholder to the Company at least 3
                  Business Days prior to the relevant payment date. If no such
                  account has been specified, payment may be made by cheque made
                  payable to the Noteholder and sent to his address as set out
                  in the register of Noteholders or, in the case of joint
                  holders, to the first named holder or to such person or
                  persons as the registered holder or joint holders may in
                  writing, received by the Company at least 3 Business Days
                  prior to the date of such payment, have directed. Every such
                  cheque may be sent through the post no later than the Business
                  Day preceding the due date for payment.

         3        CANCELLATION

                  All Notes repaid, redeemed or purchased by the Company in full
                  shall be cancelled and the Company shall not be at liberty to
                  keep the same for the purposes of re-issue or to re-issue the
                  same.

         4        TRANSFER OF NOTES

         4.1      The Notes are transferable by instrument in writing in
                  multiples of (pound)100 in the usual or common form (or in
                  such other form as the Directors may approve) to (a) any
                  member of the AMEC Retained Group, (b) National Westminster
                  Bank Plc (the "Bank") pursuant to the Letter of Credit
                  No.TFPCYF083358 or (c) any transferee of the Bank pursuant to
                  and in accordance with Clause 24.2 (Transfers by the Bank) of
                  the Facility Agreement dated 15 April 1999 between (amongst
                  others) the Company and the Bank.

         4.2      Every instrument of transfer must be signed by the transferor
                  (or by a person authorised to sign on behalf of the
                  transferor) and the transferor shall be deemed to remain the
                  owner of the Notes to be transferred until the name of the
                  transferee is entered in the Register in respect thereof.

         4.3      Every instrument of transfer must be sent for registration to
                  the Transfer Office accompanied by the Certificate(s) for the
                  Notes to be transferred together with such other evidence as
                  the Directors or other officers of the Company authorised to
                  deal with transfers may reasonably require to prove the title
                  of the transferor or his right to transfer the Notes and, if
                  the instrument of transfer is executed by some other person on
                  his behalf, the authority of that person to do so. All
                  instruments of transfer which shall be registered may be
                  retained by the Company.

         4.4      No fee shall be charged for the registration of any transfer
                  or for the registration of any power of attorney or other
                  document relating to or affecting the title to any Notes.

         5        MODIFICATION

                  The provisions of the Instrument or of the Notes and the
                  rights of the Noteholders may from time to time be modified,
                  abrogated or compromised or any arrangement agreed between the
                  Company and the Noteholders.


                                        6

<PAGE>   7




         6        DEALINGS

                  The Notes shall not be capable of being dealt in on any stock
                  exchange in the United Kingdom or elsewhere and no application
                  has been or is intended to be made to any stock exchange for
                  the Notes to be listed or otherwise traded.

         7        RECEIPT OF JOINT HOLDERS

                  If two or more persons are entered in the Register as joint
                  registered holders of any Notes then, without prejudice to
                  Clause 7 of the Instrument, the receipt by any one of such
                  persons of any interest or principal or other moneys payable
                  in respect of such Notes shall be as effective a discharge to
                  the Company as if the person signing such receipt were the
                  sole registered holder of such Notes.

         8        REPLACEMENT OF CERTIFICATES

                  If the Certificate for any Notes is lost, defaced or
                  destroyed, it may, upon payment by the Noteholder of any
                  out-of-pocket expenses of the Company, be renewed, on such
                  terms (if any) as to evidence and indemnity as the Directors
                  may require, but so that, in the case of defacement, the
                  defaced Certificate shall be surrendered before the new
                  Certificate is issued.

         9        RISK TO NOTEHOLDERS

                  All Certificates, other documents and remittances sent through
                  the post shall be sent at the risk of the Noteholder(s)
                  entitled thereto.

         10       NOTICES

         10.1     Any notice or other communication required, permitted or
                  contemplated by this Deed ("NOTICE") must be in writing and
                  delivered to the recipient by registered or certified mail,
                  return receipt requested or delivered by facsimile mail with
                  the original counterpart thereof being sent on the same
                  business day or on the Business Day immediately following the
                  date of facsimile transmission. Such Notice shall be deemed
                  received 3 Business Days after a registered or certified
                  letter containing such Notice, properly addressed with the
                  postage prepaid is posted or on the same day if transmitted by
                  facsimile mail.

         10.2     Any notice or other document (including Certificates) may be
                  given or sent to any Noteholder addressed to such Noteholder
                  at his registered address in the United Kingdom or (if he has
                  no registered address within the United Kingdom) to the
                  address (if any) within the United Kingdom supplied by him to
                  the Company for the giving of notice to him. In the case of
                  joint registered holders of any Notes, a notice given to the
                  Noteholder whose name stands first in the Register in respect
                  of such Notes shall be sufficient notice to all joint holders.
                  Notice may be given to the persons entitled to any Notes in
                  consequence of the death or bankruptcy of any Noteholder by
                  sending the same by post, in a pre-paid envelope addressed to
                  them by name or by the title of the representative or trustees
                  of such holder, at the address (if any) in the United Kingdom
                  supplied for the purpose by such persons or (until such
                  address is supplied) by giving notice in the manner in which
                  it would have been given if the death or bankruptcy has not
                  occurred. Save as otherwise provided in this paragraph, only
                  Noteholders with a registered address in the United Kingdom
                  shall be entitled to receive any notice, demand or other
                  document.

         10.3     Any notice, demand or other document (including Certificates
                  and transfers of Notes) may be served on the Company either
                  personally or by sending the same by post in a pre-paid letter
                  addressed to the Company at its registered office for the time
                  being (marked for the attention of the Company Secretary) or
                  to such other address in England as the Company may from time
                  to time notify to Noteholders.


                                        7

<PAGE>   8




         11       GENERAL

         11.1     The Register together with a copy of the Instrument shall
                  during business hours be open to the inspection of any
                  Noteholder or any person (not being a person to whom the
                  Company may reasonably object) authorised in writing by any
                  Noteholder without charge at the Transfer Office.

         11.2     The Instrument and the Notes are governed by, and will be
                  construed in accordance with, English law.


                                        8

<PAGE>   9



<TABLE>
<CAPTION>

         CERTIFICATE NO.               ISSUE DATE                 AMOUNT

<S>                                    <C>                        <C>
         1                             15 April 1999              The aggregate of (i) (pound)98,930,000 and (ii)
                                                                  amount of the Additional Consideration
                                                                  referred to in the Instrument.
</TABLE>

                      CENTEX DEVELOPMENT COMPANY UK LIMITED

         (Incorporated with limited liability under the laws of England
                         with registered number 3720116)

                           NEGOTIABLE LOAN NOTES 2001

         THIS IS TO CERTIFY THAT the undermentioned is/are the registered
         holder(s) of the amount set out below of the Negotiable Loan Notes 2001
         constituted by an instrument entered into by the Company on 15 April
         1999 (the "INSTRUMENT") and issued with the benefit of and subject to
         the provisions contained in the Instrument. Where the context so
         admits, words and expressions defined in the Instrument shall bear the
         same meanings in the Conditions endorsed hereon.

         This Certificate is evidence of entitlement only. Title to the Notes
         passes only on due registration on the Register and any payment due on
         the Notes will be made only to the duly registered holder.

<TABLE>
<CAPTION>

         NAMES OF HOLDER(S)                 AMOUNT OF NOTES

<S>                                         <C>
         AMEC Finance Unlimited             The aggregate of (i) (pound)98,930,000
                                            and (ii) amount of the Additional
                                            Consideration referred to in the
                                            Instrument.
</TABLE>


         SIGNED AND DELIVERED AS A DEED
         by CENTEX DEVELOPMENT COMPANY UK LIMITED
         ACTING BY

         Director /s/ [Illegible]          Director

         DATED:       15 APRIL 1999

         NOTES:

         1        The Notes are repayable in accordance with the Conditions
                  endorsed hereon.

         2        This Certificate must be surrendered before any transfer,
                  whether of the whole or any part of the Notes comprised in it,
                  can be registered or any new Certificate issued in exchange.

         3        The Notes are transferable in accordance with the conditions
                  endorsed hereon and this Certificate must be lodged together
                  with the instrument of transfer (which must be signed by the
                  transferor or by a person authorised to sign on behalf of the
                  transferor) at the Transfer Office.

         4        A copy of the Instrument is available for inspection at the
                  Transfer Office at all reasonable times during office hours on
                  any Business Day. Noteholders shall be entitled to receive a
                  copy of the Instrument and its Schedules without charge on
                  application to the Company.

         5        No interest is payable on the Notes.


                                        9

<PAGE>   10





                                 THE CONDITIONS

         1        REPAYMENT, PURCHASE AND REDEMPTION

                  Unless previously repaid, redeemed or purchased by the Company
                  the amount of the Notes held by each Noteholder shall be
                  repaid by 11 am (London time) on 30 March 2001. Payment of the
                  amount of the Notes shall be in the manner specified in
                  Condition 2.

         2        PAYMENT

                  Payment under the terms of this Instrument shall be effected
                  by crediting on the relevant payment date the account
                  specified by the relevant Noteholder to the Company at least 3
                  Business Days prior to the relevant payment date. If no such
                  account has been specified, payment may be made by cheque made
                  payable to the Noteholder and sent to his address as set out
                  in the register of Noteholders or, in the case of joint
                  holders, to the first named holder or to such person or
                  persons as the registered holder or joint holders may in
                  writing, received by the Company at least 3 Business Days
                  prior to the date of such payment, have directed. Every such
                  cheque may be sent through the post no later than the Business
                  Day preceding the due date for payment.

         3        CANCELLATION

                  All Notes repaid, redeemed or purchased by the Company in full
                  shall be cancelled and the Company shall not be at liberty to
                  keep the same for the purposes of re-issue or to re-issue the
                  same.

         4        TRANSFER OF NOTES

         4.1      The Notes are transferable by instrument in writing in
                  multiples of (pound)100 in the usual or common form (or in
                  such other form as the Directors may approve) to (a) any
                  member of the AMEC Retained Group, (b) National Westminster
                  Bank Plc (the "Bank") pursuant to the Letter of Credit No.
                  TFPCYF083358 or (c) any transferee of the Bank pursuant to and
                  in accordance with Clause 24.2 (Transfers by the Bank) of the
                  Facility Agreement dated 15 April 1999 between (amongst
                  others) the Company and the Bank.

         4.2      Every instrument of transfer must be signed by the transferor
                  (or by a person authorised to sign on behalf of the
                  transferor) and the transferor shall be deemed to remain the
                  owner of the Notes to be transferred until the name of the
                  transferee is entered in the Register in respect thereof.

         4.3      Every instrument of transfer must be sent for registration to
                  the Transfer Office accompanied by the Certificate(s) for the
                  Notes to be transferred together with such other evidence as
                  the Directors or other officers of the Company authorised to
                  deal with transfers may reasonably require to prove the title
                  of the transferor or his right to transfer the Notes and, if
                  the instrument of transfer is executed by some other person on
                  his behalf, the authority of that person to do so. All
                  instruments of transfer which shall be registered may be
                  retained by the Company.

         4.4      No fee shall be charged for the registration of any transfer
                  or for the registration of any power of attorney or other
                  document relating to or affecting the title to any Notes.

         5        MODIFICATION

                  The provisions of the Instrument or of the Notes and the
                  rights of the Noteholders may from time to time be modified,
                  abrogated or compromised or any arrangement agreed between the
                  Company and the Noteholders.


                                        1

<PAGE>   11




         6        DEALINGS

                  The Notes shall not be capable of being dealt in on any stock
                  exchange in the United Kingdom or elsewhere and no application
                  has been or is intended to be made to any stock exchange for
                  the Notes to be listed or otherwise traded.

         7        RECEIPT OF JOINT HOLDERS

                  If two or more persons are entered in the Register as joint
                  registered holders of any Notes then, without prejudice to
                  Clause 7 of the Instrument, the receipt by any one of such
                  persons of any interest or principal or other moneys payable
                  in respect of such Notes shall be as effective a discharge to
                  the Company as if the person signing such receipt were the
                  sole registered holder of such Notes.

         8        REPLACEMENT OF CERTIFICATES

                  If the Certificate for any Notes is lost, defaced or
                  destroyed, it may, upon payment by the Noteholder of any
                  out-of-pocket expenses of the Company, be renewed, on such
                  terms (if any) as to evidence and indemnity as the Directors
                  may require, but so that, in the case of defacement, the
                  defaced Certificate shall be surrendered before the new
                  Certificate is issued.

         9        RISK TO NOTEHOLDERS

                  All Certificates, other documents and remittances sent through
                  the post shall be sent at the risk of the Noticeholder(s)
                  entitled thereto.

         10       NOTICES

         10.1     Any notice or other communication required, permitted or
                  contemplated by this Deed ("NOTICE") must be in writing and
                  delivered to the recipient by registered or certified mail,
                  return receipt requested or delivered by facsimile mail with
                  the original counterpart thereof being sent on the same
                  business day or on the Business Day immediately following the
                  date of facsimile transmission. Such Notice shall be deemed
                  received 3 Business Days after a registered or certified
                  letter containing such Notice, properly addressed with the
                  postage prepaid is posted or on the same day if transmitted by
                  facsimile mail.

         10.2     Any notice or other document (including Certificates) may be
                  given or sent to any Noteholder addressed to such Noteholder
                  at his registered address in the United Kingdom or (if he has
                  no registered address within the United Kingdom) to the
                  address (if any) within the United Kingdom supplied by him to
                  the Company for the giving of notice to him. In the case of
                  joint registered holders of any Notes, a notice given to the
                  Noteholder whose name stands first in the Register in respect
                  of such Notes shall be sufficient notice to all joint holders.
                  Notice may be given to the persons entitled to any Notes in
                  consequence of the death or bankruptcy of any Noteholder by
                  sending the same by post, in a pre-paid envelope addressed to
                  them by name or by the title of the representative or trustees
                  of such holder, at the address (if any) in the United Kingdom
                  supplied for the purpose by such persons or (until such
                  address is supplied) by giving notice in the manner in which
                  it would have been given if the death or bankruptcy has not
                  occurred. Save as otherwise provided in this paragraph, only
                  Noteholders with a registered address in the United Kingdom
                  shall be entitled to receive any notice, demand or other
                  document.

         10.3     Any notice, demand or other document (including Certificates
                  and transfers of Notes) may be served on the Company either
                  personally or by sending the same by post in a pre-paid letter
                  addressed to the Company at its registered office for the time
                  being (marked for the attention of the Company Secretary) or
                  to such other address in England as the Company may from time
                  to time notify to Noteholders.


                                        2

<PAGE>   12



         11       GENERAL

         11.1     The Register together with a copy of the Instrument shall
                  during business hours be open to the inspection of any
                  Noteholder or any person (not being a person to whom the
                  Company may reasonably object) authorised in writing by any
                  Noteholder without charge at the Transfer Office.

         11.2     The Instrument and the Notes are governed by, and will be
                  construed in accordance with, English law.



                                       3