Garrett-Cox to leave Alliance Trust Investments in March

* Follows broad changes prompted by activist investor

* Garrett-Cox had already stepped down from board

* Investment oversight duties to be split

By Simon Jessop

LONDON, Feb 15 (Reuters) - Katherine Garrett-Cox is to leave investment firm Alliance Trust after a shareholder rebellion last year saw her forced to accept a diminished role in a wide overhaul of the company.

Garrett-Cox and the board mutually agreed she would step down as chief executive of its fund arm, Alliance Trust Investments, on March 11, with responsibilities for running its 5 billion pounds ($7.24 billion) in assets reassigned.

The move caps several months of change for Alliance Trust after Garrett-Cox ceded defeat in a high-profile battle with activist U.S (Other OTC: UBGXF - news) . hedge fund Elliott Advisors, which wanted sweeping changes to turn around a prolonged period of underperformance.

Those changes included cutting costs, selling out of non-core investments such as commercial property and changing the company's board structure.

The latter had already resulted in Garrett-Cox stepping down from the Alliance Trust board and Chairwoman Karin Forseke resigning. The decision to part company was an acknowledgement of Garrett-Cox's changed role, Alliance Trust said.

"Alliance Trust is undergoing significant change to improve both its operating performance and investment returns," Chairman Lord Smith of Kelvin said.

"Alliance Trust is now moving swiftly to implement the changes announced last year which are designed to enhance shareholder returns. This process is well underway and it is clear to us all that the role of chief xxecutive of Alliance Trust Investments has changed significantly."

($1 = 0.6902 pounds)

(Editing by Jeremy Gaunt)