I think using UHT milk is wrong, I think the Ultra High Temperature treatment kills the milk dead. Use a thermos flask to put the mixture into overnight and it cools slowly and sets right. Use a hand mixer to fully mix the culture with the milk, that ensures the spores are well distributed. Home made yoghurt is always thin & runny, but still very good. Store bought yoghurt has lots of thickeners in it like gelatine and always heaps of sugar. Adding milk powder into the mix at home helps to thicken it somewhat, but I fear unless you use gelatine it will be thin.
