Eventually found a Hazard switch in the 4th scrappy I visited .. but it made no difference at all
Still not sure about the difference in voltage measured across the plug, so started doing comparisons offside/nearside, results being that the -ve line resistance of the offside plug is 3 times that of the nearside, so a bit of temporary wiring and it worked fine if an additional line was attached to the -ve line at the plug and connected directly to the chassis.
So the "bad earth" idea seems to be correct... well done to those who suggested it some while back !
I have now wired in a semi-permanent "bodge" in order to allow the car to be driven legally, but I'm not happy with it.
Haynes wiring diagrams show line 438 as the -ve line but gives no indication of where this attaches to any "earth" point on the chassis, or if it continues within a loom right back to the battery terminal. The fact I get a high resistance seems to indicate to me that there is no "break" in the line, but probably EITHER a loose connection or a corroded terminal somewhere.
As it is unlikely, IMHO, that Vx would give a dedicated earth point to a single bulb, the question is, what else is trying to earth via the same point, and so might also fail ?? or try and "back feed" through the indicator -ve up to my "bodged" earth point ?? which could overload the indicator wiring ??
Does anyone have any idea where the "earth" point for the indicator wire might be found ??