Ntl sales staff are much like sales staff for any company- yep they will do pretty much anything to get a sale... because sale = commission
The letter sounds like it is a notice from a debt collection agency, but before it gets to that stage you usually get a number of warnings/ final notices etc.
If you wish to resolve this matter with ntl, please PM me your name, account number and a contact phone number and i will pass these to our contacts to resolve this.
If you wish to pursue this through the courts, then good luck to you... but tbh unless you know enough about law to be happy to defend yourself, or you can afford a solicitor, then i would recommend sorting this out with ntl directly.