What's happening, isn't it adjusting back with up with a screwdriver? You don't actually need the motor to work for the MOT, so long as it can be adjusted manually. Usually, what happens to the height adjuster is that the really weedy teeth on the gear (the one you put the screwdriver into) round off, so you can't adjust it with the long screwdriver.
Our mk1 was like this when we bought it, and the offside headlamp was way out. However, you can access the adjuster from the wheelarch, and there's a hexagon on the back of the adjuster that you can put a small socket on - can't remember the exact size, but it's around 6mm IIRC. Add on a ¼" ratchet, and you're away. My motor still worked, but that shouldn't make any difference. I popped the wheelarch fixings out for access, took it to my MOT man, and I twiddled until he gave the thumbs up.
The motor/adjuster assembly is easy enough to change, once you get the headlamp off, but it's very fiddly to do it on the car when you can't see the adjuster mechanism inside the headlamp, you have to engage the ball on the end with a slot in the mechanism in the headlamp. Extending the adjuster fully out first helps, and a small inspection mirror and a torch will be useful.
Hope this helps.