The moves, which take effect immediately, end the current round of senior management changes at the discretionary investment manager based in Chichester following its management buyout in 2017.

Mr Cook, who is currently director of marketing and business development and is a well known industry commentator, will add client services to his brief. Mr Marriage will continue to head Thesis’ investment committees and the firm’s Lymington office.

This news follows a decision from client services director Tony Gammon to step down from the board following 23 years service. Mr Gammon, who joined Thesis in 1980, will continue to lead the Brighton office in the interim while the business looks to find a successor in the near future.

David Tyerman, chief executive at Thesis Asset Management, said: “I am delighted Lawrence and Giles are joining the board to help us achieve our ambitious growth plans. The two of them have been integral in helping to grow the business in their time at Thesis, and we fully expect them to be excellent members of the board.

“I would also like to thank long-standing colleague Tony Gammon for his service on the board. He is a valuable member of staff, so we are naturally pleased he will be remaining with us.”

Thesis has £15 billion of funds under management and administration and invests on behalf of private clients, charities, pension funds and trusts. It works mainly with professionals in financial advice, law, accountancy and tax.