Citroen C3 back pressure through water reservoir cap.

Thermostat should have been checked when doing head gasket , likewise bleeding cooling system if needed . Possibly water pump but also head if not been pressure tested before refitting . If in doubt , carry out a block test .

Probably simplest thing to check first would be as suggested your thermostat could be checked ensure it’s opening alough you should be able to feel this through pipe work as engine heats ,also ensure that no airs trapped in the system ,

hi,cheers.just adding.whilst running engine,the top n bottom hose feel empty of pressure.