[Offgrid]

Thanks Mr Beast. It looks as if I'll have to go back to basics then.
I've swopped the battery for another known good one, and, of course, I've run the car with two different working fuel pump relays. I plan to go over all the earth connections to try to resolve the battery issue. (12.8v in the battery, and 14.7v from the alternator).

I guess the answer is to list all the sensors, and check them on by one. Maybe look at the fuses as well. Plugs and leads are pretty new, but i'll do the night-in-the-woods test to see if I can see any coronas.