Yeehaa! I did the yogurt again and it worked this time (used the UK recipe I posted earlier in this thread).
2 things were different to last time:
a) husband didn't pick up thermos flask and ask what it was (had florescent DO NOT TOUCH sign on it this time) and
b) the live yogurt culture used was more reliable.
Not sure which it was that made the difference, but one or both of them did.
It was slightly thinner than I prefer, so I made some of it slightly runnier still, mixed it with bramble jelly and turned it into drinking yogurt. The rest I've strained to become Greek yogurt. Next time I'll add 3 tblsp milk powder rather than 2, and see if that gives me the texture I like.