Thanks everyone - had a very fraught day!! Vet rang at 11, they had had a nightmare getting bloods out of him, and his ALT was double what it should be, so the vet asked if I Was willing to take the risk with the dental. I agreed, as the alternative was letting him go, so she put some fluids into him to be on the safe side. he had to have 9 teeth out, so no teeth left at all now, and not willing to eat, so if no better by the time I get home tomorrow, he will have to go back to the vet. He is dribbling a bit of blood and looks very doped - they gave him an extra strong painkiller before leaving to be on the safe side, but it is vitally important he starts to eat, as obviously not eating can make the liver worse. He had put some weight on though, which surprised me as he feels thinner - still underweight though. He isn't allowed his cat milk till the morning.
MM - vet didn't want to give him anything without having liver/kidney tests in case there were issues there, or that it might mask what is really going on. I was fairly sure his kidneys were fine as he hardly drinks, but didn't know about his liver. Vet doesn't feel his thyroid is high enough to warrant medication but a retest of his liver in about a month is advised - that shoudl be fun, he has fought them both times for his blood so far!!