INDEPENDENT financial advisers Thomas Spencer and Associates has selected a panel of the world's top fund managers to take over the investment management decisions of its Lyndhurst Managed Portfolio and Wellington Performance Portfolio. The move was announced in a letter to clients from Mr David Thomas, managing director of Thomas Spencer, following the poor performance of the funds over the last 12 months. He said the performance had caused ''great concern to us and our clients''. Thomas Spencer has taken the following action as from January 1: Lyndhurst Managed Portfolio renamed Thomas Spencer and Associates Ltd Balanced Portfolio, managed by Fidelity, Schroders and GAM. Wellington Performance Portfolio renamed Thomas Spencer and Associates Growth Portfolio managed by GT Management, Bankers Trust, Barclays and Cresvale. Mr Thomas said the Pottinger Preservation Account would remain unchanged, for the time being, with Richmond Asset Management as the Investment Managers.