I broke it
Flooded road vs the M57... through no real fault of its own - the M57 lost.
As they say... an attempt was made. And what I didn't consider (or really think enough about) is that the engine ECU is still basically wedged in place upside down - I haven't made an enclosure for it. Being upside down is definitely the worst orientation as it turns out... because any water that is thrown up into the engine bay just makes its way right down the connectors and inside...
Had a friend appear with some supplies. Lots of contact cleaner and being blown off with the airline from the back was called for. Meanwhile... why not do a bit of cleaning under the bonnet while the ECU bakes for 10 minutes at 32c in the rescuing (sorry) Jeep.
After quite a few attempts.... it finally started up and would keep running... and relief was had.