***** and violation of mandate signed when taking a contract

I had a contract with Cell C that had expired, paid R298 and have now upgraded with a mandate to pay +-R400 I guess because the new contract was never communicated to me. This month I was charged a whooping R626 and I'm being told it's October bill which for as much as I know it was paid in October and I have proof. Cell C have violated the mandate we agreed on by ridiculously charging extra money they cannot justify and since the mandate was compromised, this contract is nullified too and I don't want the contract anymore, this is day light **** and taking advantage of customers because you have a mandate authorization over them which still is violated. I was sceptical on taking the contract due to the very same experience I had with Cell C in my +10 years of consistent double contracts but this time I've had enough!