Wesbank
Incorrect information recorded

I purchased a vehicle from Agenbag Motors Klerksdorp. Wesbank processed the incorrect bank account details. I found out on the 7/03/13 that my debit order had not been processed. I paid the money cash and now interest has been charged for a mistake of Wesbank. I was informed that the payment was also loaded CASH by two Wesbank personnel Martina and client information and the Wesbank accounts department inform me it is a debit order.
Please can Wesbank inform me what to do, as I am not willing to pay the interest as Wesbank made incorrect entries on the contract. The cash and incorrect banking details.