Overheating problems

Hello. have a 2015 Meriva 1.4 turbo auto that failed on the motorway the first time I drove it any distance. the engine management cut the engine due to a lose of coolant through a small pin sized hole in the header tank. aa filled with water and i drove to a safe place where he found the leak did a snffer test to check the head gasket and found it ok, he added a K-seal just in case then had it shipped to a local garage. They replaced the header tank, the thermostat, pump and serpentine belt. then had it shipped back to me, but it is now overheating again. I tried flushing the system with no luck and then took to local garage who placed the rad. when I went to collect it they told me the head gasket was blown, I decided to take it home and look at it myself. rad was cold and when i tried to bleed it there was no water present and the header tank was empty. filled the header and it would still not bleed so there is a valve on the coolant flange which when i pulled it out a big gust of air then fluid. does anyone know what this valve is and what it does? any other suggestion for why it is over heating? see photo for where the valve fits in the bottom left corner of the coolant flange