This afternoon I made up a test adapter for the pressure switch - M10 x 1.0mm female to 1/8" BSP Male with a female cone in the end.
Assembling the switch to a test pump and guess what - the switch is INNOCENT - apply pressure switch closes release and switch opens.
I was just packing up and processing the photographs when I realised that there was a possible / probable miscarriage of justice - looking at my photograph of the Fluke meter it was on its next to HIGHEST ohms range not it's lowest

Of course I'd dismantled everything and put all away, but there was nothing for it, there would have to be a re-rial in the light of this new evidence. I put everything back together repeated the test and OK the accused WAS innocent, and released from custody.
. . at least I now have a spare pressure switch.
As for the original fault, I believe that the copper wires had corroded where they were crimped - possibly due to the sulpher in the oil, or maybe other causes, and the outer plastic sheath had held the bits in intimate contact, often adequate and sometimes not. Remember that the plastic sheath is retained by the outer crimp on these spade lugs.