LONDON (Reuters) - Lansdowne Partners, one of the world's best-known hedge fund managers, said that Paul Ruddock is to retire as chief executive to focus on his work in the arts and charity sectors. Ruddock, who ranks among Britain's richest financiers, co-founded Lansdowne in 1998 with Steven Heinz and has been a major donor to the arts and to Britain's ruling Conservative party. The 54-year-old, who doesn't make investment decisions in the firm's funds, will hand over his responsibilities at the end of June but will remain a significant shareholder, the firm said in a statement on Monday. ...