overdraft interest rate

it never seems to amaze me that I always have a problem with standard Bank. <br> <br> my interest rate on my overdraft ha's been changed from +6% to +18% this is 3 time's the value it was before. this is all done without my concernt as I do not remember signings any contract to agree to this amendments.<br> <br> how is it possible that they can change such pertinent information without letting the customer know? and the reason why it's been changed to such an amount? <br> <br> I have honestly grown tired of standardbank. this is getting rediculous.<br> <br> can they please send me a letter showing me where I signed in agreement to this amendment.<br>