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                                                                    Exhibit 4.13

                          LLOYDS TSB CORPORATE
                          City of London
                          Lloyds TSB Bank plc
                          PO Box 18436
                          39 Threadneedle Street
                          London                      Telephone: 020 7670 3733
                          EC2R 8PT                    Facsimile: 020 7670 3769
                                                      Reference: MC

The Directors             Date: 16th January 2006
VTL (UK) Limited
Incubon House
Wick Road
Egham
TW20 0HR

Dear Sirs,

                                AMENDMENT LETTER

We refer to a letter dated 21st October 2005 (the "Facility Letter") and to the
security documents required to be effected thereby from time to time. Pursuant
to the foregoing we agreed to make available to you a revolving loan of up to
(pound)5,750,000 (the "Facility").

We hereby confirm that, provided the preconditions set out below have been
satisfied prior to 31 January 2006 (the "Effective Date") the following
amendments shall be made to the Facility Letter with effect from the Effective
Date:

1.    for all purposes the Facility Letter shall be read and construed as if the
      amount of the Facility therein provided were (pound)7,000,000 (seven
      million pounds sterling) and not (pound)5,750,000 (five million, seven
      hundred and fifty thousand pounds sterling) and all references to
      (pound)5,750,000 shall be deemed to be references to (pound)7,000,000, and

2.    the definition of "Expiry Date" in clause 1 shall be amended to read:

                "Expiry Date means 30th April 2006".

3.    the definition of "Facility Limit" in clause 1 shall be amended to read:

                "Facility Limit" means (pound)7,000,000 subject to any
                cancellation or other reduction thereof within the terms of this
                letter.

From the Effective Date the Facility Letter and this letter shall be construed
as one agreement and all references in the Facility Letter to "this letter",
"herein", "hereof", and terms of similar effect shall, unless the context
otherwise requires, be deemed to be references to the Facility Letter as amended
by this letter.

All other terms and conditions expressed in the Facility Letter remain
unaltered.

As a precondition to the above amendments becoming effective the Bank is to
receive in a form and substance acceptable to the Bank:

(a)   a copy of this letter duly signed by you in acceptance of the amendments
      contained herein;

(b)   a certified copy of your board resolution approving the terms of this
      letter and authorising a person or persons to sign on your behalf the
      acknowledgement appearing on the duplicate copy of this letter;

(c)   if not already in our possession, duly authenticated specimen signatures
      of the person(s) so authorised;

(d)   such evidence as the Bank shall require to confirm that the principal
      amount of the guarantee detailed in Clause 4.2 of the Facility Letter has
      been increased to (pound)7,250,000; and

(e)   an arrangement fee of (pound)12,500.

Yours faithfully,
For and on behalf of Lloyds TSB Bank plc
/s/ Mark Jones
Mark Jones

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                                       2

                                                   Continuation of a letter from
                                                   Lloyds TSB Bank plc to:
                                                   VTL (UK) Limited

Relationship Manager

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                                       3

                                                   Continuation of a letter from
                                                   Lloyds TSB Bank plc to:
                                                   VTL (UK) Limited

We hereby acknowledge receipt of your letter dated 16th January 2006 of which
this is a duplicate and confirm our agreement to the amendments therein
contained, pursuant to a resolution of the board dated 20th January 2006. Please
debit the arrangement fee to our current account number 2524060.

For and on behalf of VTL (UK) Limited

Signed by    Lucy Woods   (name)             Stuart Blythe    (name)

/s/ Lucy Woods (signature)                   /s/ Stuart Blythe (signature)

    20th January 2006   (date)                 20th January 2006      (date)

THIS LETTER CREATES LEGAL OBLIGATIONS. BEFORE SIGNING YOU MAY WISH TO TAKE
INDEPENDENT ADVICE.