just get a new cog and chainring.
unless you have some cash to spend or there is something wrong with the cranks and bottom bracket.
That's the rational answer and if all I was chasing was chain wear, you'd be spot on.
In this case, the old crankset is probably a little flimsy and the bb has done a lot of work - it's rebuildable and has been rebuilt a few times. I have no doubt that there's a lot of life in her but I'd like to start fitting some new bits over time. The frame needs a repaint too but I don't want to lose the original stickers so will have to find someone who can make me a new set.
You're right, I don't NEED a new crankset and bb, but there's some emotional justification for it and if I'm going to do it, now's the time to do it.
Richard