Brothers of the Head by Brian Aldiss
Another excellent Aldiss novel. This one is about two brothers who are siamese twins and one of them has another (third) dormant shrivelled, creepy, grey-haired head on his shoulder. they're basically boggers from norfolk and their dad sends them off to london to be rock stars but that doesnt really work out because they fight all the time. eventually they return to norfolk where they revert to their almost feral pre rockstar existance and the dormant head starts waking up and causing trouble.
Huh, I didn't realise that was a book. I enjoyed the film.