This does not sound correct, because if you were paying off capital with any over-payment you make, your interest repayments should immediately adjust downward as the interest rate is applicable to a lower outstanding capital balance. In addition, given that any over payment is non-refundable, and it's not being applied directly to your repayments, where is it going?
There is something fishy going on here. I would ring up and clarify this point Cavangirl, especially since these funds are apparently 'non-refundable'