Weird lighting - 1987 90

When I put the main beam on the main front light on the passenger side goes out and only comes back on with the main beam off. It appears that the vehicle is an ex military 90 - could this have a bearing on the bulbs etc?). Does anyone have any experience of this and possibly a solution?

Probably a bad earth somewhere.

Sounds to me like the high-beam filiment is blown. Unlike modern cars, on a LR when you change from High to Low & visa versa the other filiment goes out hence the lack of light from that unit.
MOD headlamps are different to civy units and use a 40/45W bulb (I think???) but first you have to check if it has been fitted with a halogen conversion or even sealed beams.