unauthorised contract ( advertised as a competition )

I was contacted by dstv in november last year about a competition to win a mercedes benz ending in march this year and the only condition of entering the competition was to change my payment plan from eft to a debit order. I asked the consultant whether this would be an extra cost to me and she confirmed that I would not pay anything extra for entering the competition and that the amount to be debitted from my account would be the standard premium rate of R 874.00. I was even told by the consultant that it was not a contractual agreement and that it was on a month to month basis. The consultant even assured me that I could cancel the debit order at any given time if i wanted to default back to my original payment option. The debit order would officially go off at the beginning of december and eventually did at the standard amount of R 874.00. But at the beginning of January dstv debits my account with a ridiculous amount of R 2551.00. When I called in to find out what it was for I get told that it was for a dstv price-lock plan for 24 months. To date I have not being contacted by dstv whatsoever to resolve this issue.