You are always entitled to a second opinion from another garage if you think you have been unfairly treated. Then if necessary, go to the ministry of transport. (M.O.T) I did this once and got results, although my circumstances were different. I bought a car with a 12 month ticket, but half the things that are mandatory did not work. I reported it and the officer from the 'M.O.T' re-MOT'd the car with the original inspector present. The officer also found rust that would have failed the initial MOT as well as a badly blowing manifold, and rear exhaust silencer. The M.O.T inspector who issued the certificate got his ticket pulled and I still ended up with a 12 month ticket, although I did get the work done.
If you think the garage that failed your car was unjust in its actions, to try and get business from you, you are entitled to get a second opinion or, if you know your car is in satisfactory condition, you can formally report the MOT station to the Ministry of Transport.