Missed payment virgin media’s fault

I informed virgin media I was moving house last September, at first they wanted £350 for cancellation fee but I couldn’t pay this so agreed to continue the contract but to do this they said they had to set up a new account and they would wave the cancellation fee, which they did although it took months of stressful phone calls to try get anyone to understand and being passed from pillar to post

after this was setup they sent me a £40 fee for not returning equipment, which I was now using in a new address!! I spoke to them on 05/12/24 and was informed they would wave the fee as they could see it was an error and credit my account £40 so as far as I was concerned that was it settled. Fast forward to today and I have received a missed payment on my credit score for £40 which virgin media are saying I haven’t paid. I’ve had it confirmed twice today that they’ve credited the wrong account in December and just not bothered to tell me i still apparently owe £40 on my old account and instead ruined my credit score just when I’m applying for a mortgage. The phone lines take ages to get through on, and when you do the person is usually offshore and English is a second language. I raised a complaint and was told someone would call me (they never did) then just sent a generic email saying complaint was resolved. I’m literally going crazy with the stress of all this and don’t know what else to do I really need the missed payment removing as it was virgins fault in the first place!! 

Please help