The Duchess of York, dubbed "Her Royal Idleness" by British tabloids who accused her of preferring play to work, went back on duty at a dinner in London to start a fund-raising campaign for British museums. As royal patron of an organization called Museums Year, she was guest of honor Tuesday at the dinner attended by rock star and charity aid promoter Bob Geldof; the editor of The Times of London, Charles Wilson, and other celebrities. The 29-year-old duchess, who as Sarah Ferguson married Prince Andrew in 1986, looked slim and elegant.