Mercedes Benz Finance - Debit order - Unauthorised

My vehicle was settled with MBF and in July already and I am still receiving statements that my account is in arrears. I had to reverse last month's payment because the settlement was not processed! I had to send the settlement letter via e-mail - which they should have as the contract reference number was included on the proof of payment - how does your reconciliation department do their jobs? I have sent numerous e-mails but no one is giving me feedback. How is this going to affect my credit report and what type of service are you providing to "exiting" clients. I am really disappointed and now becoming frustrated with my time being wasted to resolve this matter.