Thank you Emerald Green, Lizzie and Taz for all your advice and information. I will definately be asking the vet at his next appointment about extra fluids and if there is anything available over here in the UK. Even if there isn't a vetinary equivalent I will ask if there are ones I can purchase anywhere else. I have taken a lot of comfort in all you've said and am determined to give Henry the best care he can have within the limits we have. It has made me feel very positive about his future.
The vet was quite negative about it, though I suppose it's because they feel they have to prepare you for the worst.
I now feel so much better about his future and trying to keep his disease as balanced as we can.
Thank you everybody, and special smelly kisses from Henry!