Hello Ian,
The push button mechanism is fairly simple in operation and to work on once you get to it. I recommend purchasing a manual Rover or Haynes to make life with a P6(B) easier and it will boost your confidence to see the pictures of this mechanism before you tackle it.
To remove the inner door panel, slide the "wood" out of the black plastic backing (catch the spring washers) to reveal the screws to remove the black plastic slider. Use a wire hook made from a piece of coat hanger to pull the spring clips from behind the door handle and window winder (not as easy as it sounds until you've done it once or twice). Unscrew the arm rest. Unscrew the pull/push button that locks the door from inside. Prise/pull the trim panel from the door and peel back the polythene sheet for access to the lock. There is a screw adjustment with a locking nut that should be obvious when pushing the outer button. Good Luck. Reverse the process for reassembly except that the spring clips may be replaced on the inner door handle and window winder prior to sliding back onto their respective square-section shaft and pressed into place with the palm of the hand.
Regards, John.
