Thank you for your question.
This should not affect your credit rating and this happens a lot, not that will necessarily be of consolation to you. I aware that debt collection agencies send out letters but are not good at responding to letters and phone calls.
Write to both Virgin and the debt agency using recorded delivery.
1. Tell them their claim is denied.
2. Tell them to supply you with the paperwork you have requested and point out that without it they have no claim against you. They have to establish their claim against you. It is not for you for you to prove anything.
3. Tell them you are relying on their representation that the account was closed and there were no further charges to pay.
4. Email copies of your letter to the Chief Executive of Virgin Media at [email protected]
Also scan and email to him the nuisance letters you have been getting about this. Bombard him with all the paperwork.
5. Do not be intimidated and take a proactive and direct approach.
I hope this helps. Please leave a positive response so that I am credited for my time.