I have no idea how he did it, but he seems to have a hunk of flesh hanging off his tail I can see the bone! I've just spoken to the emergency vet, and she recommended leaving him til tomorrow, unless he seemed in need of pain relief.
Astonishingly, he seems very chipper and is frisking around as usual and gobbling his food, so I think I'm going to follow the vet's advice and take him in first thing tomorrow.
Bloody hell though It looks very gruesome.