It's back on the road now. The man from the garage came and jacked it up and attached a booster pack and the doors opened. Jump start followed. Drove it round for half an hour this morning, and it restarted OK this afternoon. As a precaution, I've left it unlocked overnight, so if the battery dies, at least I'll be able to jump start it off my son's car (as long as it doesn't get stolen).
It's going in to the garage later in the week for them to see if they can locate the problem. Don't think they'll find it, but I've been given the name of a auto-electrics specialist who should be able to track it down, although I'm not sure I can afford to have it done.
He should be able to sort out my CD-changer as well - can't get the magazine out, but everything else works fine. I'm fed up with having to listen to the same five discs for about a year now

For the record, now that it's got power back, the driver's door and boot locks work fine with the key, although the latter needs a drop of oil. Glad I didn't use extra force with the key.
I think I'm going to get a couple of heavy duty wires soldered to the starter motor terminals and bring them out to somewhere convenient to attach a battery to - probably under the wheel arch. Then next time it dies, I can inject enough power with an external battery to get the doors open. The man from the garage thought this was a good idea.

Any comments from here?