What rights do I have to reject a car purchased in June 2010?

When we purchased the car we had agreed a timescale for the main dealer to complete the work, which they did. But subsequently the car has been returned to the dealership on four further occasions and we have now lost confidence in the vehicle. Are we entitled to reject it?

At the time of purchase the dealer gave is a letter stating if the fault was not satisfactorily rectified they would provide us with another car of the same spec.