Seems like problem on the inertia switch's circuit as you well suspected, it controlls the coil of the main relay R9, check fuses F1 and F13(engine bay), swap relay R9 with R8 to rule the relay out, if still no joy remove R9 and bridge the contact side of it(perpendicular cavities) if this makes the trick check what's missing on the coil , it should get earth from the ECM when ignition is turned on and should have permannent live through the inertia switch
My friends call me Fery
2000 Discovery Td5 ES manual
Hawkeye v.6, Nanocom EVO,
VDM UCANDAS, oscilloscope
if you think my english is peculiar it means you didnt hear my japanese
ALL MY ADVICES ARE BASED ON MY OWN KNOWLEDGE AND MY WAY OF UNDERSTANDING THE DESCRIPTION AND OPERATION OF SYSTEMS... I'M NOT A LAND ROVER TRAINED SPECIALIST JUST AN ADDICTED ENTHUSIAST.