Because it's a building society I'm paying into it's not actually my account number that I make my payment to: I make a payment to a sort code and account number which stands for the entire Nationwide, and my own account number is just the reference number on the payment. This makes me nervous in the first place, but I've done it before and it worked OK, so I thought I ought to have no trouble setting it up again.
Only when I put those details into HSBCs "new standing order" dialogue on their online banking system it complains every time that the details are incorrect, without giving any detail as to the problem. So I gave up after the 3rd attempt, and rang their phone banking instead (which incidentally wants my date of birth as part of the security procedure in ddmmyy not ddmmyyyy format, which confused me briefly). I explained what I was trying to do to the person on the other end of the phone, gave her all the details, and then realised she'd gone and processed it as a one-off payment rather than a standing order, despite the fact "I'm trying to set up a standing order" was the first thing I said. So I got put on hold for 5 minutes while she set up a standing order to start a month later, since that one-off payment had already been set up.
Now I have to wait until Feb 3rd (since the 1st is a Saturday) and a few days for it to clear before I can tell if it has actually worked properly, or if I'm just transferring money to some complete black hole. *grumble* these things shouldn't be so hard to do.