For 2002-2006 R50 Cooper and R53 Cooper S Hatchbacks, and 2005-2008 R52 Cooper and Cooper S Convertibles...

This solution seems to work for most cases, but not all.  It is definitely worth a try before you go replacing the seat with big $$$... 

 Seat Back adjust

If your seat back angle adjuster suddenly stops working, try this:

  1. Kneel in the seat facing the backrest
  2. Pull both handles (backrest adjust and opposite handle) up
  3. Push the backrest with your shoulder back until you hear a 'click' (may need to hit it pretty hard to hear the click).
  4. That should reset the seat!

 

Caution: Some users report a 'sore shoulder' after performing this procedure - proceed at your own risk...

Good luck!