Thats a bummer has happened to us all
The inertia switch on early cars was subject to a recall iirc. So it possibly could be faulty.
The earth for it runs into the main loom and joins up with all the other earths for the binnacle and dash lights. They all eventually lead back to the interior main earth point behind the bulkhead carpet above and behind the fuse box relay area.
It doesn't have a dedicated earth point for the switch alone.
You could test it by removing the black/purple and black wire connectors out of the inertia switch and temporarily join them together with a jumper connector. Crank it over and listen If the pump runs, then you know its the switch at fault.
Have you tried the connection at the washer bottle multiplug btw.? That stopped my fuel pump from working there once before.
took it out found a badly corroded wire was the issue.
Hopefully a quick fix for you
Shane