hi gaynorvj
I had exactly the same problem when I had my astra its apparently very common on astras and I never did get it sorted though changing the temperature sensor on the cylinder head did cure it for a while (about 3 weeks) before it started conking out again mind you I did get the sensor from the scrappy so maybe it was faulty too then the car got wrote off so never did get to the bottom of it
by the way the car would always start in the morning when cold and was on a "P" plate