Paid up letter struggle for account 50CADC***FA3A9

I have been struggling obtaining a paid up letter for an account (50CADC***FA3A9) I paid off 5 years ago. I have called the call centre and was promised assistance but been waiting for over a week now. I have even sent statements that show that the account was paid for (even overpaid). Now I am struggling as this affects my ability to get clearance for essential credit. I don't understand why I'm recieving such a hassle for an account that had such a little credit amount.