In message , Graeme
writes

(Thinking aloud) If I run a wire direct from the battery to a 12v
bulb, then to the pressure switch, the bulb will light. Then running
the engine *should* extinguish the bulb, which will prove the oil
pressure is OK. I think.

Answering my own question, I have just tried a test lamp between the
battery and oil pressure switch. With the engine not running, the bulb
lights. Start the engine, and the bulb extinguishes, as expected (or
hoped!).

The fault must be a leak to earth, almost certainly behind the speedo,
which is a bit of a nightmare to access, due to the heater which is
directly below the speedo.
--
Graeme