Thanks everyone
An update:
Errol's been in for his checkup tonight. I've got to admit, while the vet was really nice and I feel bad for feeling this way - she really got my back up by scruffing him to get him out of the carrier, when he was headbutting the box and purring. Wasn't really necessary
It is looking like he might have kidney damage as suspected - his wee is still full of blood (apparently that should've cleared by now) and he's lethargic and druffy (although he's been druffy since we got him). It's not a definite yet, it may well be temporary.
He goes back in a couple of days to get the results of his wee test (God was THAT a palaver) and to check on him again. In the meantime he has antibios as a just in case measure.
He is NOT allowed to be neutered at least for a while - they want him to have kidney function tests when his cystitis has cleared and then we can go from there. I dont know what that means if he does have kidney failure, that's something we'll have to weigh up (as will the outside vs inside side of things).
We discussed urethrostomy and she said there was another operation that is much more effective and we'll have to consider that should his problems persist. However, that operation is a much more major op - it involves breaking his pelvis (does anyone know of this op?). He also won't be able to have this if he has lasting kidney failure, so for now we have to try and control this in other ways.
It's not normal that he's had two episodes of penile obstruction so young, and the general feeling is that this is related to the electric shock. I asked about diet - she said she could write a prescription for us (fab as we can buy online then). She also said that it wouldn't be necessary if we fed him wet food, but when we explained that he's had this twice and has a mix of wet and dry, she recommended permanent prescription diet to be safe. I know the diets marketed for struvite, but could really use info on the ones for oxalate (in case he has oxalate) so I can look them up and know what we want when we go back for the prescription.
Thanks in advance for any advice you can give us!