In message ,
harry writes

You need to determine which side of the bulb is earth and replace that
wire which runs between that (earth) side of the bulb and the oil
pressure switch. & see what happens.

Harry, I am fairly confident I have eliminated a problem with the engine
itself. I have run a wire direct from the battery to a bulb, and from
the bulb to the oil pressure switch terminal. With the engine off, the
bulb lights, as expected, earthing through the switch. With the engine
running, the bulb extinguishes, indicating the oil pressure is
sufficient to operate the switch and therefore break the connection to
earth.
--
Graeme