If you've a Landrover Discovery 1, doesn't seem to matter which engine (200TDI, 300TDI, V8) and your headlight keeps falling out, there is a simple repair and a cheap part number to sort it out.
Get yourself some of these… its part number STC3368.
You need three of these for each light, or just check the broken ones and replace them. They are cheap, so I've replaced all three for a nice snug fit. Pull out your headlight and you will see three plastic clips in three corners of the headlight. It's a simple bit of screw driver action to take the old ones out and put the new ones in. Don't over tighten them as they could crack your plastic headlight unit – and you don't want to have to replace that.
When your done, just push it back in place. Remembering that it may be a little tight now. Shame this sort of thing isn't included in my Land Rover Discovery Petrol and Diesel Service and Repair Manual: 1989-1998 (Haynes Service and Repair Manuals).