redcrx is right.
My O2 contract was due to run out in March, I phoned up and explained that I wanted a better deal or I'd no longer stay with them, when they got funny saying they couldn't offer anything better I asked how much notice they needed or if the contract would be cancelled as I wasn't going to renew, they said it ran on and would only be cancelled if I spoke to their 'department' that deals with cancellations. Although it's bad it also stated in my renewal contract (needless to say they gave me what I wanted!) that my contract will automatically renew itself at the end of term unless I contacted them to cancel it.
Good luck.