* Sorted! (Well, partially!) *
Out early this morning to try and get to the bottom of this once and for all!
I dicided to concentrate on the rear near-side light fitting.
With the stop/tail bulb removed I checked for voltage on the yellow/black wire feeding the tail light.... 6.2v!
I checked on the off-side feed... 0v!
When I popped the bulb back in, the voltage dropped to near zero, but as Jonny mentioned, there probably isn't enough oomph there to light the bulb filament.
So, attention then turns to the rev counter again!
With the rev counter unplugged, zero volts on the tail light feed and only 25mA draw on the battery!
I shouldn't have just relied on the visual inspection of the rev counter PCB.
So, does anyone have or know where I can get a decent secondhand one?
I do remember reading on another posting (will have to see if I can find it!) that its a particular chip that causes the problem, that can be replaced... I'm quite handy with a fine tipped soldering iron (21 years as a TV engineer still occasionally has it's uses!) so if available, I'd have a crack at that!
Anyway, thanks for everyone's input and here's to keeping me old (but nice) 'Q' on the road!