The nylon gear wheels and cams inside these locking units are simply not strong enough - another example of cheap parts being used to build a so called quality car! The door units are not servicable though I did manage to take the one on the Tourer's tailgate apart and replace the broken nylon cam with one I cut and filed from a piece of aluminium - has worked fine ever since!.
As for the doors you'll obviously need to remove the door card so expect to break some of the retaining clips and have some spares standing by. In the centre of each door card there is also a large spring clip which breaks very easily if you are not careful.
To get the units out from the doors you have to remove the three screws on the door shut face and disconnect the linkages inside - very fiddly - there is also a short connecting rod (about an inch long) which is just a push fit between the back of the door handle and the locking unit - this will disappear into the depths of the door shell
Removal is also easier if you detach the vertical window runner and push it to one side.