<PAGE>
                                                                    EXHIBIT 4.28


                              Dated 1st April 2000







                                ENZACTA GROUP PLC

                                       AND

                                CHIMAERON LIMITED










                         ------------------------------

                         CONSULTANCY SERVICES AGREEMENT

                         ------------------------------














                              ASHURST MORRIS CRISP
                                Broadwalk House
                                 5 Appold Street
                                 London EC2A 2HA

                               Tel: 0207-638 1111
                               Fax: 0207-972 7990

                                      MGC/



<PAGE>

THIS AGREEMENT is made on 1ST April 2000

BETWEEN:

(1)      ENZACTA GROUP PLC (No. 3729344) whose registered office is at 5 College
         Mews, St Ann's Hill, London SW182SS (the "COMPANY").

(2)      CHIMAERON LIMITED whose address is 8 Centre One, Old Sarum Park,
         Lysander Way, Old Sarum, Salisbury, Wiltshire SP4 6BU ("CHIMAERON").

THE PARTIES AGREE AS FOLLOWS:

NATURE OF ENGAGEMENT

Upon and commencing from the date of completion of the acquisition of the entire
issued share capital of Kymed GB Limited by the Company, the Company will engage
Chimaeron to procure the engagement of Professor Tony Atkinson ("Professor
Atkinson") as a consultant who will provide consulting services to the Company,
such services to be as the board of directors of the Company (the "BOARD") may
assign from time to time in accordance with the Company's business needs and
which shall include the role of Scientific Director.

CONSULTANT'S FEES

The fee is at present L80,342 per annum exclusive of VAT, or such other
rate as may be agreed from time to time payable monthly in arrears by bank
credit transfer. This agreement shall be terminable by either party giving to
the other not less than six months' prior written notice.

OTHER INTERESTS

Other than set out in Schedule I to this agreement or as agreed with the Board
Chimaeron and Professor Atkinson shall not without the prior written consent of
the Board during the continuance of this agreement be concerned, engaged or
interested either directly or indirectly in any capacity in any other trade,
business or employment which is in competition with the business of the Company
in the Field as may be agreed with the Board provided that (other than any
shares held in the Company) Professor Atkinson shall be permitted to hold not
more than 3% of any class of shares or debentures or other securities (other
than shares, debentures or other securities of the Company) which are quoted or
dealt on the London Stock Exchange or the Alternative Investment Market or any
other recognised investment exchange (as defined in section 207 of the Financial
Services Act 1986) or enterprise investment scheme or similar fund.


                                      -1-
<PAGE>
SECRECY

Professor Atkinson shall not, except as authorised or required by Professor
Atkinson's duties or obliged by law, reveal to Chimaeron and any person or use
for their own purposes or for any purposes other than those of the Company any
of the trade secrets, secret or confidential operations, processes or dealings
or any information concerning the organisation, prospective business, business
methods, systems or affairs, finances, transactions or affairs of any company in
the Company, or any similar information in relation to any customer or supplier
of the Company, which may come to his knowledge during and as a result of his
employment and shall keep with complete secrecy all confidential information
entrusted to Professor Atkinson and shall not use or attempt to use any such
information in any manner which may injure or cause loss either directly or
indirectly to the Company or their respective businesses or may be likely so to
do. This restriction shall continue to apply to information or knowledge which
is ordered to be disclosed by a Court of competent jurisdiction or otherwise
required to be disclosed by law.

NOTES OR MEMORANDA

Professor Atkinson shall not during the Appointment make, otherwise than for the
benefit of the Company, any notes or memoranda relating to any matter in the
Field or concerning the Company's dealings or affairs, nor shall Professor
Atkinson either during the Appointment or afterwards use or permit to be used
any such notes or memoranda otherwise than for the benefit of the Company. All
such notes and memoranda made by Professor Atkinson shall be the property of the
Company and left at the registered office of the Company upon the termination of
the Appointment.

INVENTIONS

Any discovery, development, invention, process, design, improvement in
procedure, computer program, copyright works, trade name or trade mark made,
discovered or created by Professor Atkinson in the course of his duties for the
Company in the Field either alone or with any other person ("Inventions") shall
forthwith be disclosed by Professor Atkinson to the Company and shall belong to
and be the absolute property of the Company or such Associated Company as the
Company may nominate for that purpose provided that this clause shall not apply
to his rights in Inventions which cannot under applicable law be vested in the
Company ("Consultant's Rights").

If and whenever required so to do (whether during or after the termination of
his appointment) Professor Atkinson shall at the expense of the Company or its
nominee:

         (a)      apply or join in applying for patents, registered designs,
                  trade marks or other similar protection in the United Kingdom
                  or any other part of the world for any Inventions and execute
                  all instruments and do all things necessary for vesting the
                  said patents, registered designs, trade marks or other similar
                  protection when obtained and all right and title to and
                  interest in the same in the Company or such other third party
                  as the Company may require absolutely and as the sole
                  beneficial owner; and

         (b)      sign and execute all such documents and do all such acts as
                  the Company may reasonably require in connection with any
                  proceedings in respect of such applications and any petitions
                  or applications for revocation of such patents, registered
                  designs, trade marks or other protection.

Professor Atkinson hereby irrevocably appoints the Company to be his attorney to
act in his name and on his behalf to execute and do any such instrument or thing
and generally to use his name for the purpose of giving to the Company the full
benefit of this provision and a certificate in writing signed by any director or
by the secretary of the Company that any instrument or act falls within the
authority



                                       -2-
<PAGE>
hereby conferred shall be conclusive evidence that such is the case as against
Professor Atkinson in favour of the Company and any third party.

Should the Company, in its sole discretion, so require, the Inventions shall
not be made the subject of patents or similar protections, but shall be worked
by the Company and/or any associated Company as secret processes.

NON-COMPETITION

Chimaeron and Professor Atkinson acknowledge that the Company is the beneficial
owner of certain intellectual property rights relating to the Field, that all
information relating to and details of the intellectual property rights owned by
the Company in the Field and the Company's technology and business are
confidential to the Company and that it would in practice be impossible for
Chimaeron or Professor Atkinson to be involved in the Field following
termination of this agreement without employing the confidential information and
intellectual property rights of the Company. Accordingly, it agrees that it will
not and that it will procure that Professor Atkinson will not:

         (a)      for a period of six months after the Termination Date whether
                  on its or his own account or in conjunction with or on behalf
                  of any other person directly or indirectly whether as
                  principal, partner, employee, agent, shareholder or otherwise
                  howsoever carry on or be engaged or concerned in any business,
                  trade or employment in the United Kingdom in the Field;

         (b)      for a period of six months after the Termination Date whether
                  on its or his own account or in conjunction with or on behalf
                  of any other person directly or indirectly whether as
                  principal, partner, employee, agent, shareholder or otherwise
                  howsoever carry on or be engaged or concerned in any business
                  trade or employment in North America in the Field; and

         (c)      for a period of six months after the Termination Date whether
                  on its or his own account or in conjunction with or on behalf
                  of any other person directly or indirectly whether as
                  principal, partner, employee, agent, shareholder or otherwise
                  howsoever carry on or be engaged or concerned in any business
                  trade or employment in Europe in the Field.

Chimaeron will ensure that Professor Atkinson further undertakes with the
Company that for a period of three months after Termination Date, Professor
Atkinson will not without the prior consent of the Company, whether by himself
or his employees or agents or otherwise howsoever and whether on his own behalf
or for any other person, firm, company or organisation, directly or indirectly:

         (d)      solicit business from or canvass or otherwise have dealings
                  with any person, firm, company or organisation in connection
                  with the Field who or which at any time during the Relevant
                  Period was a customer of any company in the Company in
                  connection with the Field or who or which at the time of the
                  termination of his employment was negotiating to do business
                  with the Company, in connection with the Field, in each case
                  with whom or which, during the Relevant Period, Professor
                  Atkinson or any other employee of the Company reporting
                  directly to Professor Atkinson has had personal dealings in
                  the course of his employment.

         (e)      solicit or entice away from or endeavour to solicit or entice
                  away from the Company any director or manager Or senior
                  employee or salesman employed by the Company at the date of
                  termination, whether or not such person would commit any
                  breach of his contract of employment by reason of leaving the
                  service of the Company;



                                      -3-
<PAGE>

         (f)      employ or otherwise engage in any business in competition with
                  the Company any person who has during the continuance of his
                  employment hereunder been employed or otherwise engaged by the
                  Company and who by reason of any such employment or engagement
                  is or is reasonably likely to be in possession of any
                  confidential information or trade secrets relating to the
                  business of the Company or the customers of the Company.

Professor Atkinson covenants that in respect of any other company in the Company
in the business or affairs of which Professor Atkinson shall at any time during
the Relevant Period have been materially concerned or interested that Professor
Atkinson will perform and observe in relation to each such company the several
covenants set out in this provision and that each covenant shall be construed
and enforceable as a separate covenant in relation to each such company.

These restrictions are entered into by the Company and Chimaeron after having
been separately legally advised.

Each of these restrictions contained in this provision is intended to be
separate and severable. In the event that any of the restrictions shall be held
void but would be valid if part of the wording thereof were deleted such
restriction shall apply with such deletion as may be necessary to make it valid
and effective.

For the purposes of the Appointment:

"FIELD" means all aspects of Enact Pharma plc's programmes as referred to in the
Enzacta Group plc Merger and Placing Document of March 2000, including
particularly all aspects of drug and delivery systems in the fields of oncology,
osteoporosis and human nerve growth factor, its receptors and associated and
related molecules as may be amended and agreed from time to time by the
company's future programmes;

"RELEVANT PERIOD" means the twelve-month period ending on the Termination Date;

"TERMINATION DATE" means the date of termination of his employment with the
Company.

GENERAL

Professor Atkinson is not subject to any particular disciplinary rules or
procedures but should conduct himself in a thoroughly professional manner at all
times.

This agreement sets out the entire agreement of the parties in relation to the
engagement and is in substitution for any previous contracts of employment or
for consultancy services between Chimaeron or Professor Atkinson and the Company
and any associated company which are deemed to have been terminated by mutual
consent.

No amendment or addition to this agreement shall be effective unless it is in
writing and signed by both parties.

The termination of this agreement will not affect such of the provisions of this
agreement as are expressed to operate or to have effect after termination and
will be without prejudice to any accrued rights or remedies of the parties.

The validity, construction and performance of this agreement is governed by
English law.

All disputes, claims or proceedings between the parties relating to the
validity, construction or performance of this agreement are subject to the
exclusive jurisdiction of the High Court of Justice in England and Wales (the
"HIGH COURT") to which the parties irrevocably submit. Each party



                                       -4-
<PAGE>

irrevocably consents to the award or grant of any relief in any such proceedings
before the High Court and either party is entitled to take proceedings in any
other jurisdiction to enforce a judgement or order of the High Court.

Any notice to be given by a party under this agreement must be in writing in the
English language and must be delivered by hand or sent by first class post or
facsimile transmission or other means of telecommunication in permanent written
form (provided that the addressee has his or its own facilities for receiving
such transmissions) to the last known postal address or appropriate
telecommunication number of the other party. Where notice is given by any of the
prescribed means, it is deemed to be received when, in the ordinary course of
that means of transmission, it would be received by the addressee. To prove the
giving of a notice, it is sufficient to show that it has been despatched. A
notice has effect from the sooner of its actual or deemed receipt by the
addressee.

IN WITNESS whereof this agreement has been executed on the date first above
written




Signed by                                       )   /s/ S. P. Dickens
Duly authorised for and on behalf of            )
ENZACTA GROUP PLC in the presence of:           )


Signed by                                       )   /s/ Anthony Atkinson
duly authorised for and on behalf of            )
CHIMAERON LIMITED in the presence of:           )

                                                /s/ N. Blissett
                                                Nastaysa Blissett
                                                9 South Street
                                                Salisbury, Wiltshire
                                                SP2 7SE


                                      -5-
<PAGE>
                                   SCHEDULE 1

                                OUTSIDE INTERESTS

Third Millennium Intek Ltd
North Wales Coast Light Railway
New Sarum Enterprises
Tetricus Ltd


                                      -6-