Thanks so much for the replies.

I didn't realise this was so common. I think 3 of the tadpoles might be like this but I'm not sure since they won't keep still, but at least 2 of them have bent tails. My affected tadpoles are about 2/3 through development too. It's a relief to know that in some cases they can develop normally. I'm praying they don't end up with twisted spines. Poor little things

Geoff, I hadn't thought about that as a possible cause. I don't have anything to test my water hardness with but I just found a website about water in my local area, put my postcode in and it told me that on average the water around here is very soft (10.8mg/l calcium, it said). I also suspect that it might be down to a mineral deficiency now. Thankfully most of the tadpoles seem fine and I have 10 seemingly healthy little froglets. I hope no more get affected.