I have spent the afternoon looking over the car, and I'm getting more convinced that the French garage is sub-standard.
They found nothing wrong, but they must have fiddled. The non-start the following morning was odd. But I think they left the ignition on all the time we were away from the car. I was waiting in their reception for the car to be brought round. The rest of my party reported that the garage man went around to get my car with a booster pack.
Today I checked the coolant. I was charged for new coolant, but it doesn't look very pink, and I found that the cap on the tank was so tight that I had to get the oil filter remover to undo it!!
I do hate franchised dealerships. This one was Peugeot.
I think the engine light on the way back was my fault. The lambda connector is next to the CPS connector. I might have disturbed it while checking things trying to fix the non-start.
I phoned Joe at his place in Epsom and he suggested the engine might be flooded. Trying to start with the the throttle pedal down will cut the fuel. It seemed to work. But I checked the plugs and they were dry. But if it was flooded, it could only be a leak through the injectors. Possibly because I was 1000ft higher than the start of the journey. Then again,, I had no trouble on the way home.