Just how much had your chain stretched?
I haven't measured it, I'm afraid. I threw it straight in the bin.
I too was amazed at how well it was functioning in this state. I had been aware of a bit of - is the word grobbling? - going on, particularly when accelerating out of the saddle, so when it got a bit noisier I decided to have a look. I do keep the drivetrain pretty clean, with a weekly rub-down and check/top-up of lube.
Chainring is, I'm pleased to report, very hard steel and there's hardly any visible wear so I didn't even bother to flip it round. I might do that soon though, but I'll make sure I have time to flip it back if I find the bike doesn't like it.
The original bottom bracket (less than 18 months' use) was shot and the axle was bent, which explained the variation in chain tension which I'd been unable to remedy using the sheldonbrown whacking method.
I now have an almost silent bike again - a rather-fancier-than-necessary chain, a DA sprocket and a UN72/3 BB. I now need to find a way to stop the carradice saddlebag from creaking and knocking when I wave my posterior in the air.