The transvestite potter Grayson Perry is famous for his startling ceramics. They look conventional until you get close and see bizarre images and text, often sexual or featuring his teddy-bear. For his latest exhibition, The Tomb of the Unknown Craftsman, he was allowed to rummage through the vaults of the British Museum in London, setting his own culture and works alongside some of their ancient artefacts. He talks to Sarah Montague.