Hi There,
We had a similar experience with BoSi. We re-mortgaged in March 08 to avail of a better LTV rate, and once it the mortgage was all drawn down etc, we went about making monthly overpayments. Like you, we were told that we would incur charges for doing this. Because we had used a broker to get the mortgage, I went back to them with a very strongly worded letter, and they dealt with BoSi on our behalf and we now make montly overpayments each month, without incurring charges. We have also made one-off overpayment by cheque, without incurring a charge.
My advice is to persevere, it is possible to do it.
Wollran