So I have trawled through all of the previous threads about hazards flashing & fuel pump not running.
Engine cranks which rules out immobiliser, turning ign on unlocks doors and starts hazards flashing, lock/unlock ign on doesn't reset them only disconnecting battery does.
So I have tested for voltage at the inertia plug and only have 1.2V there.
Lifted fusebox and checked for continuity directly from C577 pin 3 and resistance is extremely high (~130k ohms) So i guess this is a short or a corroded wire or splice. Wiggled the wiring near C577 and also from the inertia plug to where the loom goes through the bulkhead and the reading stays firm so I guess tracing the loom behind the dash looking for damage would be next step?
Any suggestions or pointers on common pinch points would be appreciated

First of all check voltage at C0577/3 at the fusebox plug to make sure you have +12V there cos if you don't it is a problem in the fusebox(i presume that the fuse is 100% OK cos they can have hairline cracks) . If that's ruled out check resistance from the switch to C0605/3 too and if that's OK run a new wire from 577 to switch or if it's also high resistance to 605 run two new wires to the switch rather than dismantling the vehicle to chase a hidden wiring issue

Thanks, I will double check voltage at the fusebox plug (I only checked at the fuse) although I had kind of ruled it out by not having continuity from 557/3 to the switch. Worth checking regardless I agree

Job done and a repair I'm happy with. Inertia switch relocated to near fusebox.
Thx @sierrafery for the input

well done, i was just about to tell you to do that cos it's easyer and it has the same effect as on the bulkhead

Yeah much easier than finding new wire and running it to the other side neatly