Another O/S Headlight issue - getting 12V but not working - Earth issue?

Like Knobblys problem with his lighting I also have a K11 with a O/S headlight which isnt working.
I have checked the lamp holder and its getting 14V (new battery) when I use the chassis as an earth. However the lamp does not work. It has woked previously and has also blinked into life after a quick disconnect from lamp, lead waggle and reconnect (also this technique is no longer working) which suggests its a wiring fault somewhere.

Both lamp holders are OK. Car is pretty corrosion free and only 60K miles on it so everything is pretty clean under bonnet. I have cleaned the lamp holders connectors and ensured they are making good contact to the bulb itself.
During investigation I discovered a curious fly lead from the O/S headlight going to the N/S headlight which was giving it (the N/S) power (they had cut the original N/S power wire).
I checked the original N/S wire and it also had 14V so have reconnected the original wire and removed the spurious fly lead. This 'fix' is working although I have yet to check full beam, however I am no better off as the N/S has always worked!

Can anyone tell me where the most likely earth points are for the O/S headlight? I am aware of the main one under the battery (not yet cleaned up) but would imagine this would affect both lights if in a state of decay?
Would the fact that the O/S circuit was (until I disconnected it) providing power to the N/S circuit have overloaded something further up the wire - like the relays?
Please help - winter rolling in, clocks going back etc... Thanks. Andy
 
OK, fixed the non working side (new bulb (ahem)) but now the near-side has gone. It would probably work if I reattach the jumper wire from one side to the other but Im keen to get it working without a bodge if possible.
Judging by the way the side lights come on ie one side ok the other side slowly, it looks like an earth problem. I will investigate the underside of the fusebox next as thats the only place left untouched.
 
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