Car won't won't engage start relay..Key turns & dash lights up but

first place to look is the inertia switch… the manual Says its behind the kick panel in the passenger’s foot well.
well I can’t find it…

Any body got any idea where it might be in a 93 XJ40???

Try drivers side

Yea, I should have been a bit clearer.
I’ve almost striped the interior apart, even removed the driver’s side door panel & I’ve not spotted it.

The electrical guide (S65_93.pdf) clearly states…
Inertia Switch
Driver’s Footwell
Above Dead Pedal

Did you find the Inertia switch above the dead pedal (LHD drivers side, has plunger type activating device). Then if drivers foot post (LHD) A-5 fuse and no cracking for the entire fuse block (kind of common) then ignition relay behind/beside the radio. Also bad solder joints in the module behind the glove box need to be looked at or reflowed if needed. Take a look at this topic it my help. Almost everything has suddenly gone dead - #22 by ProQuest

It’s a red thing out in the open by the pedals

A picture is worth a thousand words:

This shows the inertia switch in a tripped state (button popped up, red band showing). To reset press the button down.