Cat update! →[More:]I managed to cat-proof the oven properly, so he took to hiding under the spare bed. He continued growling and hissing when approached, but I took the route of ignoring this and approaching him anyway, and it turns out he's a complete softy - loves head-bumping and chin-scratching in particular.
Now he will approach me for fussing in the evening, and will meow if I ignore him. Quite a change. We missed the vet deadline but that's not so important, compared to having a happy sociable cat!