there is another reason for this which i have just had. The door (drivers in my case )wont open unless you jiggle and yank on the handle hard several timesthe inside mechanism works perfectly
What I did was break the door lock down into 3 parts
1 the locking mechanism itself
2 the internal mechanism
3 the external mechanism
my thinking was that 2 + are both attatched to 1 there for in my case 1 + 2 worked there was a fault with 3
As part of the external locking system there is a
plastic rod with a spring round it which pushes on a lever which in turn pulls a metal rod to undo the lock that final rod is threaded so all i had to do was disconect it spin it in 5 or so turns and reconnect. Works perfectly.
I must say it is a very fiddly job too