Poor start up when cold?

My Grand Scenic 04 plate starts up in the morning as long as I have my foot on the gas. The car rumbles for about 5-10 seconds then idles ok. If I don't put foot on accelerator then it stalls straight away. After the car has been started first time in the morning, this problem is solved for the day and the same problem comes back once the car has not been started for a number of hours or a day.

Any ideas on how to rectify this problem?

AA have failed to diagnose and so has my local garage.

Thank you