Thanks for your further thoughts.

1. I haven't tried an independent earth but I have tested the earth in the plug and it's showing 0.0 ohms to one of the engine cover bolts.  I did clean up the earthing point near the inertia switch last night.
2. Thinking further on my suspicions of the relay I realise I've not ben thinking clearly.  Any measurement I take with the relay in circuit is invalid as by the time I've turned the key and connected the multimeter the initial priming phase will be over.
3. I've uncovered the hinge wiring from about 2" from the loom to 2" to where it goes in to the boot lid and it looks very good.  I've also pulled the wires and none are broken
4. Pumps are not expensive and this one sat for long periods of time (700 miles in 6 years prior to my ownership) so I've decided to change the pump.