You are not being charged for a MAC code. If you continue to use your present connection till 26 March you will have paid 12 payments and will have received one month free. In other words you have had 13 months of BB and will have paid for only 12. There is no charge for the MAC code. If you leave early and have to pay the extra month thats down to you.
Regarding your contract as far as I am aware the 12 months begins when the first payment is made and you having a month free has nothing to do with it as it was not part of the 12 months of the contract.