That's brilliant! Thanks so much for all that help - it's exactly the information I was looking for. And it's pretty much as I suspected; it sounds like the women's trifold would be usable with £20 notes but they might get slightly folded over. Probably not a problem unless you're carrying quite a few £20 notes, which is sadly not a problem I have too often!
I'll have a little think about it and decide what to do.
Thanks again for your help!