I've just spoken to the actual vet. He has had to be re-catheterised and flushed through again as he is at the moment not able to wee without the catheter. The next 24 to 48 hours will tell whether he'll be able to wee normally, and hopefully we'll find out the extent of his kidney damage, which is as yet unknown. Unfortunately it's not a good sign that he's this young and has already had two near death blockages. Still feel this is linked to his electric shock as a kitten, but it's all a grey area.
I asked whether I should get hold of the prescription diet he had last time and she said that the most important thing was that he got wet food. I'm not sure that I'd feel safe having him on just normal food having seen other cats still get this problem on wet "commercial" food. I think I'd be happier with him on the prescription, but I assume that vet doesn't sell it or she'd have been peddling it. Going to get a 2nd opinion off my usual vet on Monday.
Still no idea what long term effects this will have, or if they're going to have to operate. Thanks for all your kind messages!