Sorry
@Craiger can’t help much. I got a new crankshaft position sensor but haven’t got round to fitting it (sailing got in the way). My fault has not recurred since my first post in the subject.
It is possible all the computers got their canbus knickers in a twist as I’m in the habit of inserting the fob and pressing the engine start button more or less simultaneously.
You could try a hard reset of the computers by disconnecting the battery and, give it a few minutes, short the +ve terminal cable to ground. NOT THE BATTERY! I think this is similar to a full-on reboot of all the on-board computers.