Just to round this thread up, for those interested and have a similar problem.....
After some more time spent, all lights and warnings now function correctly, at least for the time being.
Just to reiterate, the problem was: -with ign on, pressing brake pedal caused the dash reversing light warning to illuminate albeit dim, this also happened when reverse engaged. all rear lights and dash warnings WERE working correctly otherwise.
I guess there is not a single cure for everyone, but I found one or more of the following fixed this.
There was not any obvious signs of corrosion or burning on lamp units or connectors.
Cleaned the 2 earth points in boot either side - behind rear/side boot trim.
Removed the bulb holder assembly, prised around the internal joins where the phosphor bronze bulb contacts join to the steel sections with special attention to the earth contact points.
Removed and checked all bulb fitments.
Used liberal amounts of contact cleaner (Servisol) everywhere.
Cleaned the contacts on male and female contacts of the plug.
Tried to pinch together the female plug contacts so as to make better contact.
Having been through the above check list a couple of times, did have some effectat times but didn't completely cure the problem, would have done any harm.
This was not my idea, but I really think that THIS may have been the real cure:-
Using long nosed pliers, twisted the earth pin of the male connector a couple of degrees out of true (the pin the black wire goes to), I think this may have been the solution above all else! I suggest you don't overdo this though, only a very slight twist is required.
If all else fails, an interim repair would be to run an earth wire directly from the bulb holders earth plate back to the earth points in the boot, I did this during fault tracing and it immediately cured problem, it bypasses the troublesome plug and earth circuit.
Thanks to everyone for the suggestions, and hope this thread helps others.
P.S. I still didn't find where the other end of black earth wire goes to, it seems to disappear into the trim around the rear seat backs.
