It's either a broken cable or a broken clip on the locking mechanism inside the door, the thing is, it's bloody impossible to reach it yourself without removing an awful lot or losing an arm. I actually have a spare cable, but I think you need a specialist tool to get to get the unit out.
If you could remove the unit, it'd be the easiest thing in the world the repair the clip, fitting a cable can be done with the mech in situ but you really do cut your arm up.
My local specialist wanted about £180 to fix the issue.