When you say you stopped billing, what do you mean @walworth ?
If you called to cancel, and then cancelled the direct debit, then it might be there is an outstanding balance from your final bill (assuming it was cancelled).
Call o2 (on the number on the letter) or use 0800 902 0217 to call the payment management team directly.
As it is an 0800 number you can call it for free using Skype - Guide: A Guide to Skype
Please note, this is not customer services and we cannot access your account. Do not publish personal details (email, phone number, bank account).
Link to our guide on how to contact them can be found here
