Being billed for a service I am not receiving

I am being billed for the ''Fastest ADSL'' package for the past few years which is for an ADSL line that goes up to 10mb.<br> <br> I have been told by Telkom support that my current line is for technical reasons restricted to only go up to 4mb which is the ''Faster ADSL'' package. <br> <br> I am being billed for a 10mb line while I am only getting a 4mb line. There is a price difference between these 2 packages and I should be refunded for the difference in price.