I cancelled a direct debit with Genie mobile after my 12 month contract but they were really inconsistent with taking payments. Got a letter about 3 months later from a debt collection agency saying I would be taken to court for 28quid unless I paid immediately. No warning or anything, just paid it as technically in my case they were entitled to it! Got 15,000 free overseas texts!

With yours I would go to citizens advice but unless you have any firm recollection of how and when you cancelled, it doesnt look good on your part IMO.

Do you any literature on it all? Also any Terms and Conditions from them when you took the contract out?

HTH