Hi all, and thanks for your replies.
Things have turned out pretty much as I expected but dreaded: after conversations with trading standards, consumer advice and the national debtline, it turns out I don't really have a leg to stand on because I'm the one who broke the contract. If I don't respond to their letters or pay the remaining amount, they can quite feasibly take legal action and pass my "debt" onto a collection agency. D'oh! I still think its absolutely disgusting that I've got to pay for a service I'm not happy with, and could get cheaper elsewhere. I'm going to try writing a letter to "negotiate a settlement". Anyway, for anyone out there as naiive as I was, the gym isn't as much as a pushover as you might hope and if you decide you want to escape its sweaty clutches, it will come for you. I swear to God, I would rather be obese.