TimB Looking at the dash you posted a pic of, I note it has the small round red&black sockets to the lower left of the speedometer. As best I can remember these sockets were designed to facilitate the use of electrical accessories, one of which was a parking or wander light, which had both a white and red lens and could be clipped to the side of the car as required by UK law and perhaps enforced to some degree in the dim and distant past. Whether the sockets were switched or not by the "P" switch I have no idea. I seem to remember there were issues with the insulators on these sockets failing in time perhaps due to UV degradation in sunnier climates, and shorting out.
As for wiring diagram accuracy, I suspect it is wise to treat them as a rough guide, while using the BS colour code as a slightly more reliable source of circuit identification while taking nothing for granted..?
I have found a fuse wrapped up in a BL Mini loom when cut opened during fault tracing, and from a dull memory, the webmaster at GoMoG found resistors wrapped up in his EFI loom, thus it seems the potential for anomalies abound...?
Vintage Classic Car Parking Light Lamp Clip on Window Type | eBay
Seems copying and pasting links does not operate as it once did though if you copy the above line of text into your browser URL address bar, it may take you to the relevant Ebay item...?