I have been in touch with one of the best Cossie garages in the land. My car has the Sapphire style tank fitted. The coolant level should be 1/2 an inch above the bottom inlet when cold . This seems to be the answer to the problems I have been having. Head gasket looks not to be damaged. What looked to be overheating problems was nothing more than far to much coolant in the system having nowhere to go other than out of the system past the header tank cap. Regards OAP John.