ok I've done a bit of squinting at the electrical schematics in the manual and I think I have finally cracked it.
A live feed goes to one side of the leveling switch (which is a rotary rheostat)
The other side of the rheostat goes to earth
The centre tap of the rheostat feeds the variable power supply to both the headlamp levelling motors on a Blue/Red wire
I need to remove the 2xwire connector to both the headlamp motors and switch the ignition on and test I have a voltage .
if I have voltage then I need to check the earth to the motors is good.
If both the live feed and the earths to the motors are ok then the motor/s might need replacing.
If the earth is good but there is no power from the switch I need to check the 12v supply to the switch (when I can find where it comes from grrrrrrr)
It looks like it shares the feed from the headlamp washer fuse . (Diagram 10 Haynes manual)
If the motors and earths and rheostat supply is ok I'll need to check/replace the levelling rheostat or replace any defective wiring if there is a break somewhere.
Now if it will just stop raining tomorrow .........
good idea but unfortunately the MOT tester who adjusted my lights said that there was no indication that the motors were being powered.
I tend to use my multimeter for testing but I'll rope SWMBO in tomorrow and try that first