SANBS - Accounts

Absolutely disgusted by the accounts department! My father in 2019 received blood which was paid in full at the end of that month. 3years later they did not allocate one of the payments and handed the account over! We have been hounded by CREDITWORX daily yet this was a mistake on SANBS's side. We are in fact in credit, no apologies, except they will allocate the payment and send me the 0 balance account. What happens to our name and the hounding of creditors that we were wrongfully handed over to? What about the credit that is due to us? Please advise what the next steps are to hold them responsible for the mess they created.