A big thanks to everyone for their contributions and information. I’m very grateful for the interest.
I did tap the glass but alas nothing. Re a voltage regulator, even though I believed the electronics to be full fat I made a mental note to look for one if dropping the dash. Although convinced it was wiring, in readiness I checked on the price of a replacement MEE3367 control module. Still available but the price is a bit eye watering.
Anyway, I dropped the dash to check all connectors including those to the control unit as suggested by Rog.
It was then that I was able to see an additional earth above the left hand front heater vent. Because of the aluminium under tray, fuses, acres of rubber sheeting and loads of cable ties I had been unable to see it from below. So, three earths behind the dash, two as mentioned in the initial post and a third secured to a bolt above front left heater vent.
It disassembled ok but when retightening I was aware the bolt itself was moving slightly (as was the one above the right hand vent) but nevertheless the gauges worked. A eureka moment.
I know the ecu earths on a heater box bolt but I’m unsure just where the bolt heads are located for the other two earths but seemingly under the heater intake box. So, for now I have run addition earths from the existing two earth bolts to new points.
Thanks again and I hope this info is useful to anyone who has a problem in the future
