Barclaycard/ Credit report problem - HELP

sew109

Hi I had a i24 card for years with about £6k on it in August I had the money to pay it off so I called Barclaycard and asked how much to9 close the account I was told £6314 and I double checked that was all I needed to pay. I then wrote a cheque out and the guy told me that he could not guarantee that the DD payment would not be taken against the whole amount so I cancelled the DD. I then received a letter form Barclaycard saying that the account was closed end of the matter as far as I was concerned.

Then about 2 months later I logged on to see the balance of my other (small) Barclaycard and noticed that there was an outstanding balance of £46 on the i24 I paid it instantly and checked that there was nothing that had been charged against it but it was the interest from the month that I had paid off and was not told about, and then two late payment fees.

The charges are an annoyance but what is more alarming is that the credit agencies have been told that the account is in arrears and that I have missed two payments. My mortgage application is going in next week and I have been told that this could affect it I am at my wits end with Barclaycard as despite lodging a complaint straight away I have been told it will take 40 days to get a result of my complaint.

My argument is;
If I paid off £6.3k would I really have left £21 if I had known.
If my account had always been paid on time then all of the balance was paid off and a tiny amount of interest left why was I not called
If I asked for the full amount to pay to close my account and then got a letter saying that the account was closed why should I expect anything to be added afterwards.

Is there anything that I can do ? I have spoken to what I believe every available channel at Barclaycard and have been told that I will get a call back in the next 72 hours but am doubtful that they will do what I am asking.