Ok, I thought about it and then rang the RSPCA. They have collected him and taken him to a vet for treatment. The officer thought it was just mange, not a bite, so that's probably good news. If no owner is found I think there's a danger I might have to adopt him, though he's semi-feral and would probably live outside. I said I wouldn't have another cat when Dani died ... I gave away all the cat paraphernalia ... But I wouldn't want him to be put down.