Ok, the good news is...it's still there! I didn't think it had been nobbled in order to be nicked at 4am, but you never know, so I was relieved to find it where I locked it.
The bad news is....it's still there! My efforts at liberation were no more successful than yesterday's. It didn't help that the 'substitute key' I found (rather too easily, which should have raised my suspicions) turned out to be not the right key at all, but a similar looking one that wasn't. In any case, I think the problem isn't with the key but with the lock. Something in there seems to be preventing the key going in fully, so it won't turn.
The other good news is, quite by chance I bumped into a bobby on a bike, who said that as long as I can prove it's mine, they'll be happy to come down with a pair of bolt cutters and do the liberating for me. Which is very good of them, and more than I expected TBH. So, I plan to have another look for that backup key, but then I think it's going to be a call to the local nick and hopefully at least get the thing freed.