Changed the control board which made no difference. Then following your suggestion started unplugging things in turn, since there’s not not much else to go wrong.
It was after unplugging the water input solenoids that the fault went away. To my surprise, when I plugged them back in, the fault did not come back !
There may be some weird wiring issue which has been cleared for the moment by the movement of the wiring loom ? Fingers crossed