<PAGE>   1
                                                                    EXHIBIT 4.12

                                LETTER AMENDMENT

                                                    Dated as of October 19, 2000

To the banks, financial institutions
    and other institutional lenders
    (collectively, the "Lenders") parties
    to the Credit Agreement referred to
    below and to Citibank, N.A., as
    co-syndication agent for the Lenders

Ladies and Gentlemen:

         We refer to the Amended and Restated Credit Agreement dated as of
December 17, 1997 and amendments thereto dated as of April 29, 1998, May 29,
1998 and June 7, 2000 (such Credit Agreement, as so amended, the "Credit
Agreement") among the undersigned and you. Capitalized terms not otherwise
defined in this Letter Amendment have the same meanings as specified in the
Credit Agreement.

         The Borrower has proposed to acquire Perpetual plc for an approximate
purchase price of 1.05 billion pounds sterling (the "Acquisition"), which may be
paid in part by the issuance of up to 300,000,000 pounds sterling in principal
amount of notes, and has proposed to amend the Credit Agreement to permit
consummation of the Acquisition and to permit the issuance of such notes. You
have indicated your willingness, on the terms and conditions stated below, to so
agree. Accordingly, it is hereby agreed by you and us as follows:

         The Credit Agreement is, effective as of the date of this Letter
Amendment, hereby amended as follows:

         (a) Section 5.02(b)(iii) is amended by adding to the end thereof a new
     subsection (L), to read as follows:

             "(L) Debt in an amount not to exceed 300,000,000 pounds sterling
         principal amount of notes issued in connection with the acquisition of
         Perpetual plc."

         (b) Section 5.02(e) is amended by adding to the end thereof a new
     subsection (xvi) to read as follows:

             "(xvi) Investments made by the Borrower and its Subsidiaries in
         connection wit the acquisition of Perpetual plc."

This Letter Amendment shall become effective as of the date first above written
when, and only when, on or before October 27, 2000, Citibank, as Co-Syndication
Agent, shall have received counterparts of this Letter Amendment executed by the
undersigned and

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the Required Lenders or, as to any of the Lenders, advice satisfactory to
Citibank, as Co-Syndication Agent, that such Lender has executed this Letter
Amendment, and the consent attached hereto executed by each Guarantor. This
Letter Amendment is subject to the provisions of Section 8.01 of the Credit
Agreement.

         On and after the effectiveness of this Letter Amendment, each reference
in the Credit Agreement to "this Agreement", "hereunder", "hereof" or words of
like import referring to the Credit Agreement, and each reference in the Notes
and each of the other Loan Documents to "the Credit Agreement", "thereunder",
"thereof" or words of like import referring to the Credit Agreement, shall mean
and be a reference to the Credit Agreement, as amended by this Letter Amendment.

         The Credit Agreement, the Notes and each of the other Loan Documents,
as specifically amended by this Letter Amendment, are and shall continue to be
in full force and effect and are hereby in all respects ratified and confirmed.
The execution, delivery and effectiveness of this Letter Amendment shall not,
except as expressly provided herein, operate as a waiver of any right, power or
remedy of any Lender or any Agent under any of the Loan Documents, nor
constitute a waiver of any provision of any of the Loan Documents.

         If you agree to the terms and provisions hereof, please evidence such
agreement by executing and returning at least two counterparts of this Letter
Amendment to Susan L. Hobart, Shearman & Sterling, 599 Lexington Avenue, New
York, New York 10022.

         This Letter Amendment may be executed in any number of counterparts and
by different parties hereto in separate counterparts, each of which when so
executed shall be deemed to be an original and all of which taken together shall
constitute one and the same agreement. Delivery of an executed counterpart of a
signature page to this Letter Amendment by telecopier shall be effective as
delivery of a manually executed counterpart of the Letter Amendment.

         This Letter Amendment shall be governed by, and construed in accordance
with the laws of the State of New York.

                                       Very truly yours,

                                       AMVESCAP PLC


                                       By /s/ [ILLEGIBLE]
                                          --------------------------------------
                                          Title:

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Agreed as of the date first above written:

CITIBANK, N.A.,
   as Co-Syndication Agent and as Lender

By /s/ ALEXANDER DUKA
   --------------------------------------------------
   Title: Vice President

BANK OF AMERICA, N.A.,
   as Co-Syndication Agent and as Lender

By /s/ JOAN L. D'AMICO
   --------------------------------------------------
   Title: Managing Director

THE CHASE MANHATTAN BANK

By /s/ ELIZABETH H. SCHUZLE
   --------------------------------------------------
   Title: Managing Director

SUNTRUST BANK, ATLANTA

By /s/ SUSAN R. CARROLL
   --------------------------------------------------
   Title: AVP

By /s/ [ILLIGIBLE]
   --------------------------------------------------
   Title: Director

WACHOVIA BANK, N.A. F/N/A

WACHOVIA BANK OF GEORGIA, N.A.

By /s/ KAREN H. MCCLAIN
   --------------------------------------------------
   Title: Senior Vice President

THE BANK OF NEW YORK

By /s/ TIMOTHY J. SOMERS
   --------------------------------------------------
   Title: Vice President

FLEET NATIONAL BANK

By /s/ [ILLEGIBLE]
   --------------------------------------------------
   Title: Director

MELLON BANK, N.A.

By /s/ JOHN R. COOPER
   --------------------------------------------------
   Title: Vice President

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CIBC INC.

By /s/ ROBERT MENDELES
   --------------------------------------------------
   Title: Executive Director

DEUTSCHE BANK AG, NEW YORK AND/OR
CAYMAN ISLANDS BRANCH

By /s/ ALAN KROUK
   --------------------------------------------------
   Title: Vice President

By /s/ CLINTON M. JOHNSON
   --------------------------------------------------
   Title: Managing Director

NATIONAL WESTMINSTER BANK PLC

By /s/ [ILLEGIBLE]
   --------------------------------------------------
   Title: Relationship Manager

THE ROYAL BANK OF SCOTLAND

By /s/ [ILLEGIBLE]
   --------------------------------------------------
   Title: Relationship Manager

STATE STREET BANK AND TRUST COMPANY

By /s/ STEVEN G. CARON
   --------------------------------------------------
   Title: Vice President

CREDIT LYONNAIS NEW YORK BRANCH

By /s/ SEBASTIAN ROCCO
   --------------------------------------------------
   Title: Senior Vice President

HSBC BANK PLC

By /s/ [ILLEGIBLE]
   --------------------------------------------------
   Title: Relationship Manager

UNION BANK OF CALIFORNIA, N.A.

By
   --------------------------------------------------
   Title:

BNP PARIBAS

By /s/ [ILLEGIBLE]
   --------------------------------------------------
   Title: Vice President

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                                     CONSENT

                                                    Dated as of October 19, 2000

     The undersigned, each a Guarantor under the Guaranty dated February 13,
1997 (the "Guaranty") in favor of the Lenders parties to the Credit Agreement
referred to in the foregoing Letter Amendment, hereby consents to such Letter
Amendment and hereby confirms and agrees that notwithstanding the effectiveness
of such Letter Amendment, the Guaranty is, and shall continue to be, in full
force and effect and is hereby ratified and confirmed in all respects, except
that, on and after the effectiveness of such Letter Amendment, each reference in
the Guaranty to the "Credit Agreement", "thereunder", "thereof" or words of like
import shall mean and be a reference to the Credit Agreement, as amended by such
Letter Amendment.

                                       INVESCO, INC.

                                       By /s/ [ILLEGIBLE]
                                          -------------------------------------
                                       Title: Chief Executive Officer

                                       INVESCO NORTH AMERICAN HOLDINGS, INC.

                                       By /s/ [ILLEGIBLE]
                                          -------------------------------------
                                          Title: Secretary and General
                                                 [ILLEGIBLE]

                                       A I M MANAGEMENT GROUP, INC.

                                       By /s/ ROBERT H. GRAHAM
                                          -------------------------------------
                                          Title: Chief Executive Officer &
                                                 President

                                       A I M ADVISORS, INC.

                                       By /s/ ROBERT H. GRAHAM
                                          -------------------------------------
                                          Title: President