Paul, does the lock pin go up and down when you cycle the locks? If it does then its possibly your carrier. If not then it is the lock itself. See this thread.. http://www.xoutpost.com/bmw-sav-foru...nger-door.html

Or, as o.l.t said above you may need to look at the excellent write-up by BrianX54.4is.. http://www.xoutpost.com/bmw-sav-foru...-fix-pics.html

Good luck, let us know how you get on!