Mine was doing the exact same thing on MOT day! When I turned on the headlights, all things on my clocks turned off (revs, fuel, water temp).
I changed a few things, so can't pin it all down exactly for you,
- The headlight switch plug was slightly loose
- The H4 connectors on both bulbs had melted through the plastic
- Part of the main lighting loom was exposed (possible short?)
- The connectors on the base of the clocks were slightly loose
Mine was doing this intermittently just when driving about,
see here. Whacking the dash appeared to make them work again - so I reckon its obviously something loose in the dash.
Since cleaning up the loom in the engine bay and replacing wiring/plugs, I've solved my problem now, fingers crossed.