Thanks for the reply. I've been speaking to a few people I know who keep fish and they all agree with you that its a swim bladder problem and to treat it with salt.
I've been following the swim bladder advice on the Coldwater Help and Advice on these forums and have treated it with some epsom salts once so far (yesterday).
It is still having the same problems It still reacts to being touched and will swim off a bit but very quickly sinks and lies flat. It seems to have far less energy now. It doesn't seem to be moving if not prompted and I would guess it hasn't been eating since this started. I fear the worst.
Pond readings are similar today (0/0/2.5). The hair algae seems to have gone balmy at the end of last week and is everywhere.
It's fins looked fine, and there were no other marks that looked like disease. There were three small scuffed scales on the side that was always resting on the bottom, but I am putting that down to it lying on that side for almost a week.
The other fish still look fine, but we are on close watch at the moment just in case.
As sad as it is to lose the fish, I don't really feel that I have learned anything either, which is frustrating!
I'm sorry to hear that your comet didn't pull through. I suspect that there wasn't anything you could have done, but of course you had to try. If you really want to know what happened, a necropsy by a qualified vet (or even an interested amateur) might be useful. But considering that all your other fish are healthy, I don't think it's really necessary.