I had the same problem with 3 about 12 months go, they were demanding about 98 pounds even though i had informed the that i wished to terminate my contract. Every time they sent me a letter I returned it with a covering letter saying "this contract was terminated on .........", I then got a letter from Moorcroft Debt Recovery Service, who were demanding the money, they got a letter back outlining the situation, and that I would be more than willing to see them in court, and that if they continued to persue this claim that i would instigate legal proceedings for harrasement, and would send them a bill for my time and expenses.
After a week or so I got a letter back saying the debt had been written off.
these big company's annoy me, they think they can intimedate you. My advice is refuse to pay them, and tell them you will be happy to go to court.