Thanks for all the replies.
I've checked them really carefully for any skin problems/lice/mites and there is no sign of anything untoward. They love being handled - no squirming - just lots of 'chortling'/happy noises.
Today, things appear to be improving. They have been close to eachother and even though they are chattering their teeth they're not 'going' for eachother as before. I've decided to leave them as they are for now but will keep a really close eye (and ear!) out for World War 3.