I have a further update. Driving home from a long trip, I got a full Christmas tree of error lights, followed quickly by the failure of all electronic systems. ABS, VDC, parking sensors, then the radio, air con, window and door switches. Couldn't get my luggage out of the boot, because it wouldn't open either.
I got it home, checked all fuses and voltage across the battery was 11.5V, so it looked like an alternator problem. Charged up the battery, and still, a click when I hit the starter, and nothing else. Started to strongly suspect a Body Control Module failure, and to despair. Dave from TI Autos
had a cooler head though, and suggested that it sounded like an earth problem, that he'd seen the battery to body earth strap go rotten. As that's cheaper and easier to fix, I started pulling stuff out. It can be done without moving the battery tray, but it's much easier with better access.
Disconnect the battery terminals, release the battery strap nut (13mm) and make sure you have a magnetic reach tool handy, it wants to fall into the depths of the car. (Same goes for putting it back, drop it down and do the first rotation with the magnet). Disconnect 3x 10mm nuts to free the ECU tray and move it out of the way. There are 3 more nuts holding the battery tray in place, and 3x ties to cut to free the wiring conduit. The tray lifts out pretty easily.
The main negative to body earth was made almost entirely out of rust!
I used a wire brush attachment on my drill to clean out the mount, bring back the copper strap to actual copper and I swapped out the horribly corroded bolt for another cleaner one, from which I stripped the coating.
For good measure I did the nearside suspension turret earth (which goes to the loom rather than the battery negative) too.
So far so good, not a hint of failure to start, no Inertia Switch Intervened and the PD across the battery terminals on idle is 13.9V, so I should have fewer charging issues.
It costs €0 to do, though I'll find some conductive grease to cover the over-exposed parts with. The earth strap is copper under all that coating, and the body is mild steel; there'll always be galvanic corrosion, so I'll make this an annual 'service' task.
Give it a go, it could work!