It's perhaps indicative of my state of mind why I didn't put two and two together to make the answer to why Pops has had a bad tummy and skin complaints... It was, in fact, only when I stumbled across a post in a forum of mine that I discovered that peppermint tea can sometimes cause diarrhea and skin irritation in nursing babies. Peppermint tea is what I've started drinking by the bucketload for the past few weeks.
Head, meet desk.
How on earth it didn't enter my head that there may be a connection when her brother has an allergic reaction to mint I don't know, but since I stopped the tea her problems have begin to, er, dry up. I'm sure it wasn't the cause as the timing's wrong but early indicators show that there's a link. Unfortunately, that leaves me without a hot drink of choice!
ETA: I've reintroduced a couple of peppermint teas into my diet and so far so ok. On second thoughts I don't think it was that after all, but I'm not taking chances!