Patrick
a few more thoughts
1: a faulty panel lighting switch. A quick spray of electrical contact cleaner (Not WD40) might help, if you a haven't got a spare to try.
2: on the series 3 at least, there is a resistance built onto the top of the main lighting switch, which is placed in series with the panel light supply & partially dims the dash lights when the main lights are on. An open circuit there could cause a dash blackout. Is this feature on the ser. 4 and 5,? or have you got wee hoods on the lenses?
3: dash lights are not supplied via the voltage regulator, it regs only 2 gauges.
4: I've seen wiring breaks caused by "Scotch-lock"type accessory-cable snap-connectors
5: there's nothing worse than an *intermittent* fault ;-)
anyway, if your earths seem sound, I'd unscrew the main light switch and panel light switch, and check voltage at all the terminals as they hang below the dash and you operate the switches. (If I remember right, there's numbers on each switch which should correlate to the wiring diagram (or is that old Fords?)). I had to replace a panel lights switch when I overhauled my electrics at Xmas, and this switch might well be your prime suspect. I only wish welding was this logical!
You'll get it fixed, eventually. let us know how!
Andy
Will Glasgow Celtic FC win the title after 9 years of Rangers domination? Will I hear the game on the radio with all that metal grinding I'll be doing?