Terms and conditions line will always protect CarTrack

I am really dissatisfied with cartrack I installed their tracking device in November Last year and it was not working till I called them, I had 3 alarms that were ok because the system was working to my suprises we only discovered when I called them that my device was broken and I tried to get technical team to help me. I drove to their offices still recieved bad service and was forced to cancel their contract and the actual day I wanted to cancel I had to pay 2600 and after a few days someone called and we agreed on 2650 but I saw a debit order mandate that they will debit 2660 I don't know why I am paying them I want to sue the company can you give me advice because they never looked after my car and they failed me.....