Thanks for the responses, guys.

I've done other actions; moves, rotations, etc, and never had problems. The selection could have anchored itself, I suppose, but...

And I tried clicking on various options, not just 'transform selection'.

From your replies, it looks like it's perhaps not something obvious - I haven't got twenty replies explaining 'that old no-brainer cock-up again'. Still, at least I found a way to do it, that's the main thing. I'll check out the stretching again next time I need it.