Doug, thanks for the heads up on the part numbers, that will save a bit of searching, if/when I prove the switch to be at fault.
Richard, as you suggest the oil pressure switch is indeed a single pole two way unit incorporated as part of a safety circuit, designed in this case to cut out the fuel pump in the case of an accident. and in the case of my Morgan it seems one of the internal two switching actions has failed... I have yet to crawl under and confirm that the normally closed (N.C.) connection has failed, and that failure deprives the fuel pump of electrical supply while the ignition key switch is being held in the START position.
I did attempt a description of the circuit wiring for those who may be mystified by wiring diagrams though I am not at my best at 04:00hrs in the morning.
If anyone wants to kick the operational objectives and failures around a bit more to gain understanding, just SHOUT..