Okay, update:
She spent the night snoozing comfortably in the litter tray, because cats are contrary like that.
Had a bit of gunk in her eyes this morning, and was a bit sniffly. She nommed down the other half of the sachet of catfood from last night without trouble, and has had some cat milk. I spent all morning with her on my lap, and eventually gained enough trust to clean her eyes out properly - they don't look too bad, just a bit weepy. I think we both must have fallen asleep for a couple of hours (purring cats have that effect on me).
I managed to persuade her that licking a knob of butter off my precision scales was a good idea. She appears to weigh ~640g. The interwebs are surprisingly non forthcoming with a decent kitten weight chart, but it looks like that's on the low side for 8 weeks, and she could well be a scrawny 10 weeks or more, given how prominent her ribs are.
She's had several wees since last night, but not as yet passed any solids. On the plus side, she hasn't vomited anything either.
I got a call back from the cat rescue people, who were delighted that I have a potential home for her. If she's got potentially infectious cat flu (which doesn't seem unlikely) it would be even more difficult for them to take her. They don't do vetinary care other than neutering, so I've made an appointment with a local vet, and molished a bicycle-friendly cat carrier out of one of barakta's plastic crates of bisexual propaganda. I expect they'll give her some antibiotics and probably some eye ointment (which is no doubt going to be fun).
Right, I should feed myself before trip to the vet.