My credit score has been affected by false charges on my account

I changed broadband providers in late September and sent my virgin media kit back. I was then sent texts and email saying that If I don't not send the kit back I will be charged £155. I got in touch with virgin who said that they have received my kit and to ignore the messages as they are automatic. Fast forward a few weeks and my account was charged £155. I was already in nearly £50CR so virgin took that and put my account into £105 debit. 

I contacted virgin again and they put a formal complaint in on my behalf saying that it was an error and will be refunded and corrected in 72hrs time. 1 month later was sent another bill saying I still owe the money. Once again contacted virgin to have an apology and that they will correct my bill. Still never happend. Last month was sent another bill saying that I still owe the money for my returned kit and that if I don't pay they will take it further and damage my credit score with it.

Once again another complaint and an apology saying it's an error on their part and they will clear the debt on my account. When I asked for the money I was in credit with to be refunded I never heard anything back only to find that my complaint had been closed.

This morning I woke to an email off my credit score telling me that my score has gone down 105 points for the debt on my account, which virgin has said themselves that I don't owe them. I don't know what to do anymore as I've put in 2 formal complaints and had multiple conversations with agents saying it's an error on their part and they will correct it. 3 month later and nothing has happened but they have decreased my credit score.

Who do I go to to have this sorted once and for all?