Overcharged on contract

I have been billed for excessive data usage on my tablet for software updates that happened during the night on 30 August 2015. My contract was signed with Nashua but as a result of the takeover by Autopage I was automatically moved to Autopage. With Nashua I had a monthly limit and I was informed after the fact that with Autopage it is not possible. I am now being charged a bill of around R1100 compared to R299 which is my usual bill. My other problem is that on my package I have a free giganite bundle but the updates for some reason did not make use of this and was billed in rand value. This is really unacceptable and unfair. I have called the call centre who cannot assist and advised that I will have to pay.