Gyms memberships are notoriously hard to get out of, I read about it all the time on moneysavingexpert.com forum. You should post it on there really.
Unless you are happy about having your credit rating knackered for the next 6 years as a default will be issued (if not already), you need to pay it off pronto
And yes, they can take you to court with a view to issuing you with a CCJ.