Many thanks for this helpful guide.
On my Multipla, the Torx screw was so tight that the nut moulded into the headlamp body broke loose and I had to drill into the top of the screw until I could snap its head off. Be very careful when doing this, the entire 'lifebelt' housing (which fits across the body between the bonnet and the windscreen and holds the headlamps) is made of some kind of plastic / polycarbonate and it's too easy to damage it with a whirring drill bit - ask me how I know...
I had to replace the headlamp because the reflector had turned brown. I suspect that this may have been caused / exacerbated by moisture (of which the source may be water lying in the plenum chamber because the drain is blocked - these are horrible to reach and clean!).
My plan is to try and open the headlamp i.e. remove the glass cover and replace the reflector (with a suitable reflector from some other fog or driving lamp) because it is getting r e a l l y difficult to find a Multipla in a breaker yard here in Netherlands.