Ok - just to update and close this thread - found the fault!

I fitted an indicator alarm some years since which starts to beep when the indicators had been left on - easily done in a Cobra but having been a BMW driver for some years I no longer use them . I forgot about it as you could never hear it anyway other than in the garage. After removing the dash and checking the switch which was all fine I realised the alarm was not working anymore. Sods law this was in the rear battery area but when I disconnected it everything worked fine again!

So problem solved and now the dash is off it gives me chance to add a warning light for the coolant level, tidy up the wiring and remove the clock to replace with something useful.