Mtn overcharging and no replies to complaints that I have logged

Mtn has overcharged me four months now and when I go there and try to get statements to see what they are overcharging me by 5-600 a time, they tell me they can't pull statements on their "new" system. Last year they overcharged me by double and never rectified the problem. If I reverse the debit order, they cut me off immediately.

This treatment is very unjust as I have been a loyal customer of theirs for almost 20 years.