Wesbank overcharging on interest.

Wesbank is clearly not doing ethical business. We have been paying an additional R2000 on our installment since inception. Our Interest rate is linked to prime and currently at 11.25%, however we are charged interest at 51% of our capital balance and have been overcharged in interest to the amount of R35,000. We tried to get this resolved, but it is like talking to a recorder as they keep on giving the same answer. No logical explanation can be supp****. I am an accountant by profession and it is part of my work to calculate interest. We have been trying to resolve this now for about 8 weeks, without any logical response.