Your switch has probably gone into meltdown.
There are no relays on earlier Disco's, the supply is taken direct from the battery, through the column mounted switch to the headlamps.
This, despite what one magazine technical section believes, is quite a common occurrence.
Immediate fix is to replace the stalk with another LR, Metro, Montego or Maestro stalk (try a breaker yard).
Long term fix, as it will likely happen again, is to upgrade the cabling, find the supplies to the dipped and main beam and chop them off from the headlamps and use them to switch a relay (one each for main & dipped). Take a fused feed direct off battery via relay to headlamps.
The shorter length of circuit will likely improve the light output too as you will have less volt drop on the circuit therefore delivering more volts to the lamps.