Tragically bad service

My recent attempt to settle an instalment sale agreement on my MV left much to be desired with Investec. I called for a settlement letter and paid the full amount the same day, which was reflected against my account within the 7-day validity period of the settlement letter. And yet I have been charged another month's instalment, with my banker suggesting that the bank made an error on the settlement letter. They have been unable to show where this error was made or provide me with their workings. My banker has been largely unable to resolve this issue and we are now at a stalemate. I raised this issue via the Investec online dispute resolution system - I have yet to receive a reply or confirmation of my complaint. While they hold my vehicle registration certificate I am largely held captive by the bank - hardly what I would call exceptional customer service that one would associate with a private bank.