This sounds like they are scamming you, and you should ring up and again and say if they take any more money out of your account that shouldnt be taken, and if they don'tcancel the internet, they you will cancel your contract, and that they will lose a good customer.
If they keep taking money out of your account, i would cancel the contract altogether and go to another network, like O2 or vodafone, id say O2 is the cheapest,and do great deals.
It has happened to me before, and i have had to cancel the contract, read over any documents that you have, making sure that it isn't just a few months at �15 and ten it goes up to �27!
It is one thing you got to watch out for,most of these contracts have it half price for the first few months1
Sorry i coudn't help any more
Good luck!!