Ridiculous Data Charge with NO notification

My Vodacom Telephone number is ***. I requested from them: 1 gig Data a month, with top up for further use, and if it ever for whatever reason goes over even though Ive put a cap on it, they must notify me. I got a data bill last month for the amount of R 4246.98 ex VAT. I am so cross about this, because my phone NEVER stopped working, they OBVIOUSLY never set my cap despite numerous requests, and I NEVER got a message to say Ive gone over my usage. Altech has really stuffed up with this, and I REFUSE to pay this for their own stupidity. Please help me sort this out, as I CANNOT afford this. Their invoice number is: ***