Just back from the vet.
He did a sprinkle of wee on the table while we were there, so he's not blocked but not weeing normally. Vet was able to get enough on a test strip to determine there was blood in his urine, but there wasn't enough in his bladder for a sample for full urinalysis.
So could be crystals, could be UTI, either way in cats a lot of the time it can be stress related. I couldn't think of anything right away, but on the way home I remembered we'd heard a dog barking next door a couple of times in the last week, so it could be that (doesn't seem to be a resident dog however, maybe a visitor).
So I've got some gabapentin for him (cos obviously the pain of such a thing causes stress in of itself) as he can't have NSAIDs, I'm to give him that twice a day, wet food only and add water to it, if he blocks take him back in immediately, and if there's no improvement by Friday the vet wants to see him then (hopefully with enough in his bladder for urinalysis).
Fingers crossed for the auld fella...