There are cards available at the moment which are interest free for both balance transfers and purchases but not the BMI one unfortunatyely.

So far you have not received an adequate explanation for charges so you could call MBNA again, ask to speak to a manager and tell them if you do not receive a clear answer by a certain date you will refer the matter to financial ombudsman service. There is off chance this might just get you a positive result. I know from experience when I have made errors MBNA staff can be accommodating ... 'we wont charge you this time but if it happens again...'