Tonight the auction ended, with the computer bid up to £19 and me still the winning bidder. I've contacted the seller to point out that I'd requested cancellation, and again asking them to cancel the sale; they've sent me an email saying I've won it, they aren't prepared to cancel.
So I've pointed out that they can either cancel the sale now and offer it to the second bidder for whatever their final bid was, probably £18, or wait a few days and open an unpaid item case and eventually cancel the sale. That'll leave me with an unpaid item strike and probably negative feedback, but to be honest I don't really care that much. It'll be interesting to see what happens.