Breach of Contract

I have a contract with Neotel for 15 gigs a month, when taking the contract I was told that if I would like more than the 15 gigs I would need to call in and arrange it. Choosing to only have 15 gigs for financial reasons, during the duration of my contract they have allowed us to go in excess of the 15 gigs and billed me without the arrangement being made. This month my fee is 3 x the amount I agreed on when I took the contract on.