Problems with bank what should I do?

have an ongoing dispute with RBS. Around a year ago I went to a branch to close my select silver account with them. They told me it has been done and that I didn't have to do anything else.About a month ago I have received a letter from debt recovery agency showing I owe £615 to RBS. Thinking it is a mistake I went into a branch to enquire about it where I was told that my account still exists and that I am in fact £615 overdrawn. I have asked them to confirm what transactions put me in £615 overdraft. Those were charges for the account of £10 a month plus interest and other bank charges. I made a complain and had it looked at by the appropriate department. They confirmed that it is in fact a bank error and that they would remove the charges and amend my credit file which has been affected as this default is visible on my credit file for a year now. My equifax credit score has gone down to 191, I am unable to obtain any credit and I am being turned down for everything I am applying for, and above of all have been declined for mortgage. RBS has refunded the charges but failed to rectify my credit file, I have been chasing them up as they seem to be doing nothing about it. It's been over a month and they have not sorted it out, I have been given no compensation and they have not even attempted to amend my credit file yet! I have counted the amount of calls I have made over this month trying to sort it out and they have come up to 41 calls and nothing sorted.

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