Credit File Amendment

Hello,

On Check My File it is showing a missed payment to Virgin Media in March 2019. I am trying to remortgage ASAP and this is really affecting my ability to be able to do so. This is the only missed payment on the whole of my report and it is incorrect. I had finished my contract with Virgin Media and an overcharge was made but it was agreed at the time it was an error and was then taken off. I have tried to call and was given an email address but I just keep getting a message saying it will be dealt with within 21 days. I need to consolidate debts and remortgage ASAP as we are struggling but this one thing is preventing this as it is showing as adverse information on my credit report. Please can you help as I don't know where to go from here?