Virgin Media - Credit Mark removal??

Good morning everyone.

I am a newbie to this and a newbie to sorting my credit history out.

I had some money troubles around 7 years ago and have worked very hard to get myself back on track.

Having checked my credit report the only bad marker on my score is from Virgin Media in 2011. I lived with a friend for 6 months and had virgin tv installed. My bills were paid every month ontime via direct debit. I called up to cancel the service upon my moving out and asked at the time what was outstanding on my Bill if anything. They informed me that my account was clear (that was that or so I thought) I then a received a letter from my previous letting agent a couple of months later, which was from Virgin Media stating that I owed them £25. I called up to query this immediately and was informed that this sum was not due when I called up originally, as the bill had not yet been generated. But because they did not have my forwarding address (I didnt give it to them as I didnt think they would need to contact me as the account was up to date) they sent this bill to the my previous residence. I complained over the phone regarding the money, but paid it in the same phone call. No mention was ever made of it being recorded on my credit file and I am now fuming that it has been marked as defaulted. Any advice on how I can have this removed??

I don't have any paperwork to back anything up as it was all done over the phone.

Thanks in advance