Hi all,
Thanks ever so much for your replies, which are a big help. In answer to Amanda, Bertie's bladder wasn't blocked - the first sign I saw of anything being wrong was when there was blood in his urine - fortunately he comes inside to use a litter tray so I noticed straight away and took him to our local emergency vet. The vet was able to feel that Bert's bladder was empty, so no blockage, but I have been warned that this can happen and be fatal. However, since swapping to the new diet he's passing urine completely normally and the crystals are apparently dissolving, so I think he's out of danger for now.
The vet has said no more dry food, even prescription food - I have to say this has confused me though, as if cats with urinary problems are not supposed to eat dry food, why is there a dry version of the pouches Bertie is eating? Is it for a different type of urinary problem? He was previously eating a mixture of wet and dry food, and doesn't seem to be missing the dry food too much, thank goodness. In answer to the comments people have made about acidifiers, the vet also mentioned that if Bertie didn't like the prescription food, I could return him to a normal diet and give him acidifying tablets, which would be one week on and one week off. I think I'd prefer to carry on with the diet though, as Bertie hates taking tablets and I can't crumble them into food with having two cats, as I can't be sure who's eaten what!