In 1987 William Boyd wrote The New Confessions, the disarmingly plausible memoirs of a made-up man. Then came a biography of Nat Tate, the American artist who, it emerged after the launch party, never actually existed. You'd think for his next trick he might write some non-fiction, just to really confuse people, but no - you pick up Any Human Heart, the journals of a womanising writer called Logan Mountstuart, knowing it's a fake.