As a quick follow-up to that last one, the founder of the Piers Gaveston had this comment
‘It is a ridiculous story,’ he says. ‘As far as I know David Cameron was never a member of the Piers Gaveston Society, so there would have been no need for an initiation ceremony. He may well have attended one of their parties, but the pig’s head story is purely malicious gossip.'
The problem is that this is all much too late: that the whole saga will end up on Snopes in 25 years' time is probably about the best Cameron can hope for.