A very recent thread on the forums goes some way to establishing the fact that no attention is paid to written complaints sent to VM and, with that in mind, if you are 100% sure about the facts as detailed in your post (even better if you have anything by way of proof*) then I would advise as follows:
1. Write to Moorcroft using the Royal Mail Recorded Signed For service advising them that the debt is "in dispute" and that any further correspondence from them in the way of demands or any detrimental information having been recorded against your credit history with regard to same will result in you issuing a formal complaint to the FSA and OFT advising them of their breach of the terms of their licence to trade.
2. Copy that letter along with a covering letter (again by Recorded Signed For) to the home address of the Company Secretary of Virgin Media (these details are available for a small fee from
here).
In that letter you should advise the Company Secretary of the facts of the case and invite him to address the matter to your satisfaction within fourteen days - otherwise you will take appropriate action to seek a remedy through relevant third parties.
Seek his assurance, in writing, that the debt is either incorrect, or void and that he has taken the appropriate steps to have your credit history amended accordingly. You will require a copy of your credit file to satisfy yourself that this has, indeed, taken place.
*Proof can be obtained by requesting a copy of all personal data held by VM in relation to yourself and your account under the terms of the Data Protection Act. They may charge you £10.00 to provide you with the information (in fact VM may try to charge you £15.00 if you don't pay the £10.00 by direct debit
).