It was a very simple problem for an engineer to diagnose the reason.
Engine cut out at 85 mph in outside lane of motorway in heavy rain, would not restart, had to traverse slower traffic to get to hard shoulder. Very frightening I was told by the other senior engineers.
Carb found to have water in it. Only entry points air filter and pressure balancing pipe. Obviously going to be the latter. Discounted water in fuel as it was not a local area problem.
Question was why and I did not inspect the failed cars so I did not become aware that some tube lengths differed.
No one had any accident but described as very scary and we were all worried it might happen to any of us.
Tony