I once signed a contract with 3 for a mobile dongle. Because it never reached expectations as per the literature I decided to cancel it but was unable to do so. There is a plethora of numbers to ring and each contact I made went into a blind alley. To cancel you need a number and a returns envelope but I couln;t get one. In the meantime I was receiving invoices even though I cancelled my direct debit.
After about 2 months of trying I wrote a letter to one of their head offices (they have more than 1) and someone rung me up telling me to phone a specific number which I did. This started the whole process over again, All the while it costs you to phone these 0845 numbers.
Best of luck in your case!