A Shade Greener

It is now around 5 years since my boiler was fitted. The sales man told me it was hassle free, they even offer discounted other plumbing services. The contract is for 14 years at £45 a month. Today we received a letter stating we are accountable for regularly flushing out our heating system and apparently I agreed to this in the terms and conditions. The sales man did not tell me that piece of information. The blackmail comes alongside too. Should I not regularly flush out my heating system the. I will be responsible should the boiler breakdown? When did I agree to this? How is this hassle free? I explained to the sales man the £45 a month was very expensive, but he sold me the idea of hassle free, just sit back and enjoy a better heating system. I believe I have been missold and I will be seeking legal advise.