Leonardo da Vinci was perhaps the greatest artist who ever lived. Yet his reputation rests on only a handful of pictures - including the world's most famous painting, the Mona Lisa. As the National Gallery in London prepares to open its doors on a remarkable exhibition of Leonardo's work, Fiona Bruce travels to Florence, Milan, Paris and Warsaw to uncover the story of an enigmatic genius - and to New York where she is given an exclusive preview of a sensational discovery: a new Leonardo.