I measured my chain last night - 10 links were 25.8-25.9 cm. So I ordered a new one, tools and spare link pins (Shimano 8-speed HG). I'm thinking this amount of wear might explain why my least favourite gears feel and sound not quite right, while the ones I use the most are fine (i.e. the favourites are worn with the chain).
A new cassette isn't much money, but changing the cassette makes for a much bigger job, and I'd have to buy the tools. As it is, it's the first time I'll have fitted a new chain.
The bike probably has 2000-2500 miles of mostly tarmac, just enough dirty stuff to get grit everywhere, taking ~100kg up some reasonably steep hills.
EDIT: The difference between my question and this one is that the old question doesn't say anything about amount of stretch (though the answers there would tend to suggest the bigger job)
EDIT2: I finally got the chance to get a photo in daylight. I don't like the way the chain is sitting (at the top) on the sprocket it's meshed with, though the 11 and the 32 are better than I thought -- I overlaid the image of the teeth of the 28 (little used) on the 32 (works quite hard sometimes).