I decided to test the voltages by putting a breakout box between the throttle pedal and the plug this morning. All the voltages at this end were OK started the engine and it accelerated fine. Reconnected the pedal and all OK.
Was it a bad connection on the plug and the pins of the break out lead cleaned them?
Was it damp (although there was no evidence of water around the ECU) where it had been valeted?
Only time now will tell if it happens again.
Thank for the replies
Ivor