First thing to check, have you got a 12v feed from the combination switch on the Red/Black wire.
The gauge illumination earths via the same as all the other warming lights, so if you indicator / high beam work then thats ok.
If you have the feed up to the gauge cluster, then my guess is that you have shorted and burnt out the track on the printed circuit.
Wiring diagrams here http://www.manser42.freeserve.co.uk/Beat.zip if you need it.
Steve M
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Its all gone Norfolk!!!
Now in the Lotus position.