Ok, unexpected update.
It turns out snowy didn't need to be put down today after all.
He will be on daily medication for life for gout, which is apparently his main problem. I also have to give him antibiotics and the same rehydration treatment he has been having the past week or so.
The vet thinks that it is likely he has had this problem (the gout) for some time, most likely longer than the time I have had him so I am relieved to not have done anything wrong.
I go back in a fortnight to check on progress, so a much better result today than anticipated :)
Thanks for all the support, it has helped me a lot :)
John