Dont pay it and dont worry. Even if you have breached your contract, they'll lost any case they may take out against you. By law they simply cannot enforce any unreasnoble terms in the contract. Cut off any method they have of getting money from you, and change service providers. They are one of the worst in the country. Ignore any correspondance and threats. You will know if they actually do take any court action out against you, dut to the parerwork you will recieve. It is imperative that you DO reply to this.
Take this advice from a man that has taken several companies to court, and been taken to court by companies. I have never lost to the fvckers.