I took him to the vet anyway, who checked for white blood cell count, bacteria, crystals, and urine concentration. Everything was fine except the urine concentration, so I picked up a bunch of special wet food that's supposed to help with this, and brought it home. We did make a potential appointment for Wednesday morning - and decided that if he hadn't recovered by then he'd go in for further tests. But Tuesday evening he was basically acting fine, so I didn't bother starving him overnight.
At the moment he's not 100% back to his normal self, but doing dramatically better. He's had a checkup in the diary for this Saturday set up for ages, so we're going to use that to make sure that everything has returned to normalish.
One of the interesting things that has come out of this is that after three months of being nervous about the garden, he clearly decided that pressing his undercarriage on the grass would be nice, and has now wandered out on several occasions to take it all in.
Thank you all so much for saying lovely things during this. I know that for some of you "The cat is peeing blood" is a regular occurrence, but having had him almost die the previous time we were exceedingly worried, and it was lovely to have people rally around.
Here he is happily asleep with Sophia. (And my shoulder protruding slightly into shot):
The one nice thing to come out of it is that we'd had the Monday booked off for travelling back from my parents, so we put Sophia into nursery as usual and had a lovely walk along the Water of Leith, which had very high waters, and popped into the Botanic Gardens, where there was a really good exhibition with incredibly detailed huge photos of insects. If you're in Edinburgh between now and the 10th of November then I highly recommend it.
Original post on Dreamwidth - there are comments there.