Today I did the following:
Switched ECU with one from GrahamL (thank you!)
Replaced other yellow connector of death with spade connections
Checked the battery is secure
reseated every fuse under the bonnet and in the fuse box
Went out for a drive and there was no difference the car still cut out exactly as per the earlier video.
When I got back inspected the earth strap to chassis connection. My observations were:
1) The battery earth strap was next to the chassis with the gearbox strap on top of the battery strap
2) The battery earth strap had red insulation tape which was sandwiched down by the gearbox chassis strap - slightly separating the 2 earth connections. Which you can just about make out from this photo:
This was the red tape which shows it was being squashed:
and after trimming:
I cleaned up the chassis connection:
This was the condition of the original gearbox to chassis connection:
I sanded down all connections so they were shiny. It was reassembled with gearbox connection next to chassis and the battery to chassis on top of the that (opposite to when I removed it).
I ran out of time to take it for a drive however will test it tomorrow.