With all due respect i think you may have gone a little OTT with this, i agree it isnt a nice thing to happen but there are various reasons why this could have occured.
Maybe the system auto generates a letter when a complaint is raised on there system? - Again i agree a better system should be in place.
Maybe it was a new staff member who forgot the reason for the complaint? - Again more training is required
Maybe it was just down to human error - these things do happen.
I think lodging a complaint would be satisfactory but i think taking this to the M.P or the FSA is a little overboard, i very highly doubt that the company did this on purpose & they should be given the chance to review & respond to your complaint, remeber a company and a person can only get better by learning from there mistakes, If you were to complain to the FSA about this the company would be hit with a £300 bill just for the FSA fee, and in the long run its the public who will pay for this with higher insurance cost's.
Again this wasnt meant in a way to P*** you off, just giving my thoughts.