Hi Becky,
I am in the UK too, and my cat was also on Caninsulin for a few months. He wouldn't stabilise & the vet wanted to increase his dose to 6 units twice a day. Like your cat, he had lost considerable weight.
The best thing you can do for him is learn to hometest. It's the only way you will find out what effect the insulin is really having on his system. If you are unwilling to learn, then the Vet will need to do a curve on Winston & find out what's happening.
Caninsulin was designed for dogs, not for cats. Cats have a much faster metabolism & so the insulin wears off in cats very quickly. Some cats will therefore not achieve long-term stability on this insulin.
Instead of going up to such a massive dose with a fast-acting, short-lived insulin, please suggest to your vet that you switch the cat to a much longer lasting insulin. If the vet activates the 'cascade' system, they will be able to prescribe insulins that are normally for human use. A couple of us UKers on here use Bovine Hypurin PZI, that we had to persuade our vets to prescribe. This lasts much longer & will help your cat stabilise. My Milo stabilised very quickly on Hypurin, on a much lower dose than the Caninsulin, and then went into remission.
But, to be honest, whichever insulin you are using, the best thing for you to do would be to learn to hometest. We can help you if you want to?
Where are you based?
Juliet