Carphone ware house billing

I have recently upgraded my mobile to a contract of 600 minutes anytime and unlimited week end calls for 35 pounds a month. But on the 12 th or 15th of august I noticed that I am being charged and I called the customer care regarding that for which they said there has been a system error and they would sort it out once by bill is generated. So I called them on the 4 th of september as I got a bill of 189 pounds. and they said it would take around 14 days to sort it. Apart from that I also sent a mail on 12th to them .And I got a call on 13 of september from them saying that their system Is unable to calculate the bill so I am liable to pay all the charges incurred .I tried to explain to them about my contract but the person who spoke with me was very arrogant and had no interest to listen to me. Even when asked is there someone with whom I could speak with , he said there is no one who could help me and I need to pay the entire bill of 189 pounds as the system is unable to claculate and it is calculated properly from september. What should I do in a situation they are not willing to accept. I hop you could help me or guide me in this situation.

regards

Anil Doddapaneni