Thanks for your post, Ian - perhaps you're 'sheading some light' on this matter.... (sorry). Our car is 1987 vintage, one of the first de-Dion chassis, so your diagram sounds/looks accurate and contemporary.
Well, last night the inevitable happened - used the lights after a run to the monthly Carrotlands meeting and when back in the garage fully expected the problem to remain. But it didn't - the parking light on the NS was on (as was the OS) without the ignition. So - I'm happy, but no closer to understanding why....
There is a small (matchbox-sized) black, finned box plugged into a white connector which eminates from the fusebox under the dash and attached by Velcro to the inside of the scuttle bulkhead - wassat, then? Could that be a separate Dim/Dip control of the same vintage? Please forgive my lack of electrickery knowledge.
Any clues? Not so much concerned, just curious.
Cheers,
Pierson