The plastic lamp holders are old and a little perished. They do not grip the lamp as tightly as they used to. I could get new holders but if the car fails its MOT on headlight aim, then I will be replacing the headlamps anyway.
As a temporary fix, I have tywrapped the crimp connectors together so they grip the holder tighter, thus gripping the lamp tighter. That's the theory anyway. Let's see how they stand up to a bit of road vibration.