50 mph is generally a wheel balance issue or tyre fitting so here are some suggestions in order of likelihood:

  1. Tyre(s) not fitted properly on the wheel(s) - check that the bead is regular all round on both sides of all of the wheels. If not go back and swear at whoever did the job incorrectly and get it done properly for free
  2. Wheel balance, despite the fact that it has been done - if the above is the cause, the wheel(s) will need rebalancing anyway
  3. Faulty shock absorber or damping rate incompatible with the new spring rate
  4. Steering needs tracking
  5. Wheel bearings - should have showed up at the MOT
  6. Brakes grabbing - stuck or partially seized piston(s) on the front disc servo(s)
  7. Steering rack loose