I think I have narrowed down the reason not starting when cold.

After thinking about it and opening the lid on the AFM no fault found and air temp sensor worked to spec with a hairdryer and multimeter. Car starts with it unplugged as it demands maximum richness on open circuit.

So round the other side and looked at the coolant sensor which I had read is what is used by the M4.1 to decide injector on duration (richening of mix). The sensor was reading as per spec but when I examined the connector the black earth wire had been sheared in two places near the connector. Ah ha found it, so soldered in a new wire and jumped to start the car. No joy. out with the multimeter and it looks like the signal wire is open circuit somewhere as infinite ohms with the ignition on. A job for another day.......