japitts advice NOT to pay the amount is something you should heed for the time being. The onus is upon VM to provide evidence that you owed them money, that you did not pay them that money, and to prove that they issued appropriate notice or final demand to you at that time. Any credit agreement that VM provided will almost certainly be regulated by the Financial Conduct Authority, and the FCA position in these circumstances (courtesy of the excellent DebtCamel website) is "Where there is a dispute as to the identity of the borrower or hirer or as to the amount of the debt, it is for the firm (and not the customer) to establish, as the case may be, that the customer is the correct person in relation to the debt."
I would suggest you contact Citizens Advice to understand your legal rights in this situation, their service is free.
Potentially you will need to send a "prove it" letter to Virgin Media, and if they cannot provide complete evidence then you can require them to clear the default notice and pay you compensation. Given VM's dismal track record on all matters of customer service, you may need to take the matter to the Financial Ombudsman after checking with them that this matter is in scope for them.
This will take some weeks to resolve, but there is a process, just take it one step at a time.
