Chain really quite stretched, should I change cassette at the same time? 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).
The easiest way to tell is simply to install the new chain on the cassette. If the cassette is worn from the old chain, it will skip when you apply a decent amount of pressure (standing up will usually do this in your big ring and half way down your cassette). If it does jump, you'll need a new cassette. It's really as simple as that.

I would be surprised if you need a cassette. The lower model cassettes are made of steel and don't wear easily. (My current cassette, a Shimano 6700 is now 12,000 miles. I change chains about as often as you did and I do keep it well lubed).

You may notice a bit of noise when you change chains. I notice this also and it usually goes away after some mileage. If not, you can always try adjusting your rear derailleur a bit to see if that helps.