The vertical adjuster at the back of the headlamp is connected to the reflector by means of a ball-and-socket. The 'ball' is on the end of the metal adjusting bolt and the 'socket' is a plastic cup with slots to allow the ball to click into it. This plastic cup becomes brittle over time and breaks, allowing the reflector to fall forward and making adjustment impossible. I have failed to find a replacement, or be able to get the broken part out to try and repair it so I estimated the correct reflector position and glued it in place (with Araldite). So far so good and it passed its MoT, which checks lamp alignment.
Dave.