Credit score after cancelling direct debit

After 3 months of poor service (350Mbps dropped to 80Mbps) and a few engineering visits, I decided to switch to another provider.

I was lucky enough to speak to a Virgin Media operator, told him that I want to close the contract and settle all debt in full during the call. He assisted me, I made the final pay and I cancelled the direct debit.

2 months later (this November), I was called by Virgin Media, telling me that I am still due a payment, but I said that on my previous call I have settled the account in full and I'm not sure what I'm paying for, or whether my account is actually closed.

Soon after, I received a letter from BPO Collections and my credit scoring turned bad.

I have made the payment to the debt collectors, and getting worried did it again on the Virgin Media website, but the credit scoring still shows the missed payment.

This proves to be a more serious issue than I thought, as I'm in the process of remortgaging now and this stain on the credit report is really restricting my options.

I have sent an email to Creditfileamendments@virginmedia.co.uk, but they promised to get back within 21 days, which is far too late for me.

How can I clear the issue from my Credit Report?

Thanks