Thanks guys, I checked the thermostat yesterday evening, and it could be seen to be operating,{opening and shutting} with water temperature changes. I will get the gasses checked ASAP in the header tank. There are no exhaust gas type smells from the header tank when the cap is removed after running the engine. There is also a good flow of coolant from the turbo pipe when the engine is running. I am not loosing any coolant after running the engine, now that the header tank water level has been reduced. Another thought is that when coolant blows past the header tank for whatever reason on a normal car, it is not noticed wheras on a Cosworth , it blows straight out of the bonnet vents, and causes immediate panic. I have run the car in the drive again, for about an hour with no obvious problems. I will take it out later today to see if anything changes. Kind regards ,John.