The supplier cannot impose a charge without your agreement. If you disagree, phone them and tell them how much you think it should be and they will change it. However, be careful that you do not underestimate! It seems like a good idea to use the new tariff based on recent usage otherwise you will be seriously behind this time next year. The bills are in arrears. You are simply paying for estimated usage before you get the bill. They are not trying to rip you off, even if it looks like it.