A3jeroen said:

I'd premix (50/50 IIRC) or buy ready mixed and fill her up.
Do not overfill. Min cold is good. Max cold and it'll spit it out through the overflow when warming up.

gr J

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OK I'll get paraflu and mix it with water 50 50, I hope the guys in the shop will know if I can use tap water or not. Will fill it cold up to the min mark.

Thermostat it probably is. Second to that is air in system. Just filling header tank is not enough Unless the header supplies coolant to top of engine and highest point where coolant flows. But as its planned the hoses go in side of engine then coolant often goes upwards.
So is there coolant bleed valves on this engine as I'll prob need it done sometime or a hose we can crack open at the clip to release the air.