Hamuel (he's a Syrian, Snuggleham) went to the vets' this morning, and was very good. The vet says his sudden hair loss (he's going thin in a few other places too) does not appear to be mites or an allergy as he is not red or itchy, but is most likely due to a change in temperature. Apparently hammies moult their summer coat and grow a winter one when it starts to get cold, and the vet thinks this is what Hamuel started to do, but our heating just came on in the last few days so the house is now nice and warm, and he hasn't had the necessary cold to spark off the growth of his winter fur. So... I've turned down the radiator in the bedroom (he has plenty of cosy bedding) and I'm going to give some more protein food and a bit of catfood and cod liver oil too, so that should help.
As for the possible blood, Hamuel had a good check up and the vet said there's really nothing obviously wrong with him, he's very healthy and probably just excreted something red that he ate, but to keep an eye on that just incase.
He had his nails trimmed too, and was rewarded with a milk drop and a couple of pine nuts for being so good
Thanks everyone for your help and advice, I'll keep you posted. Hopefully won't have to knit him a little jumper or anything!
x