I have found an alternative way of stopping the door pin from dropping without removal of the lock mechanism or repair of the tiny spring. You just need to add a bit of friction to the door pin to stop it slipping back down after it is unlocked. I did this by putting a bit of horizontal force on the door pin with a cable tie, which i secured around some holes in the metal in the inner door, behind the door panel. I'll take some pictures tomorrow and post them.