As multimillion-pound art heists go, it's on a par with the Thomas Crown Affair. There was no shoot-out or dramatic getaway: not so much as a motion sensor was disturbed when thieves entered the Los Angeles mansion of investment banker-turned-art collector Richard Weisman. They stole a series of 11 Andy Warhol prints depicting famous sports stars including Muhammad Ali, Chris Evert and Pelé. One of Warhol's Ali prints recently sold for $9m.