Apparently the Red Cross opened some sort of account since I hadn't given them money that way before, and that plus three small transactions, two a couple of nights ago and one yesterday morning, triggered their fraud system. Apparently a lot of fraudsters do the red cross thing to check that the account details they have are genuine. Needless to say they did this while I was out shopping, so the first I knew of it was my card not working at Tescos.
No real harm done, and it got fixed in about five minutes, but you have to wonder - with umpteen millions given to charity last night because of red nose day, how the hell do they stay on top of it?