The main battery is low and battery protection is turning them off until the battery voltage comes up. Charge the main battery and that will sort itOkay. More oddness.
They have all decided to work for a few minutes on one run.
On return home they worked for the whole journey.
I can only assume a lose connection on that 20A fuse, or having removed it for a long while (minutes, maybe 10) reset something.
Thanks all!