Thanks everyone - he seems much better now, I fed him three small meals of fresh chicken yesterday as advised and he was fine (after starving him on Wednesday), today he has had a normal pouch and has polished it all off
(gave him it in two stages).
Vet thought he might have picked something up outside that he shouldn't have - VERY likely!!!
Because he had a bit of blood in his motion, she gave a steroid jab as well as an AB, and to make sure his worming is up to date. Hopefully he is ok now - I'll not dare let him out again!!