Hi Guys, having replaced the rear wheel shaft bearings and hub seals, then noticing that the hand brake cable was corroded, then finding that both rear hydraulic brake cylinders were seized, I finally got round to looking at the RPM problem.
Couldnt find any faults with the cables or connections.
I removed the Tacho and "worked" the Dip switches back and forth with some elecrical cleaner and lubricant.
Guess what, works a dream now. Happy me!!
Thanks kindly to all that assisted me in this odd fault.
PS, now converting the dash to stainless steel toggle switches with chrome bezels on the instruments. Also fitting LED illumination to the instrumentation, complete with lots of diodes and an LED Dimmer unit. Looks good.