billing and repairs

I have a contract with c track for 2 cars and one of the unit is giving me false alarm since installation (March 2014) and I have been asking them to rectify the problem and they always promising to send someone to repair the unit, 18 months down the line they still haven't sent someone and for two consecutive months I have been over charged and query about that but they couldn't help me and I want to cancel my contract with them and the saying I have to pay a cancellation fee of which I was paying for 18 months for the unit of which is not working. I am tired of their lies and I'll rather take business somewhere where I will be taken serious.