I had the exact same problem with my drivers door lock last month, and was dreading stripping down the door to get to the linkages, as they are a pain to get on and off, turned out to be the simpleist fix of all time! it was just the interior locking knob sticking down, a quick squirt of duck oil on the black plastic locking knob and job sorted! I'm not suggesting this is the same fauly as yours but gotta be worth a go?