Thankyou everyone and Burn, I am so sorry for your loss. Poor thing, 8 is so young...perhaps you're right and it progresses more rapidly in a little one.
Well I'm about to ring the vets, we're going away at lunchtime so she needs to be seen this morning.
She spent the night under ds2's bed, and yesterday though she asked for food (just fed her when I posted) she didn't eat it.
She was hungry this morning but had a couple of mouthfuls, and then just looked at me and then she drank quite a bit, so I am in little doubt that her kidneys are packing up.
She seemed uncomfortable when I lifted her round her waist, just gently, while she was standing.
Either she has some kind of infection or it's her kidneys, I think - well, maybe the most likely things anyway but only the vet will know.
I am prepared that we may lose her. She hasn't been happy for some time, and I read that despite management kidney failure will always progress and having nursed a couple of cats through serious illness I'd rather she didn't face that.
I'll update when I know more but might not get a chance today.
Thankyou again for all your help.