I have a 2lt hdi citroen which keeps pushing the water out of the header tank ive had the head+ water Pump?

You could try dropping the radiator and sticking a hose into the top hose mount to flush out any debris which may be causing a part blockage. Run the hose until the water runs clean from the bottom hose fitting. Remember to open the heater controls first to ensure that the heater matrix is cleaned also. Afterwards, flush the radiator.

does it overheat after expelling the excess water

sounds like a major air lock mate

When the cylinder head was removed do you know if it was pressure tested for cracks & machined. or did they just replace the head gasket. Sounds like the system is pressurising which could be caused by a cracked or distorted cylinder head. If the temperature is going to high it may be the cooling fan is not cutting in causing local boiling and forcing the water out. Hope this helps.

There could be three answers to your problem that I can see: 1. As has been said before you have crack in your engine block or head (it's not impossible but not so common) 2. The gasket between the engine block and the cylinder head is damaged. Both 1 and 2 means that pressure from cylinders enter the water circuit 3. The heat is very high (over 100 degrees; subjective depending on environmental pressure) and water becomes air which rises the pressure in water circuit. Try looking first at water temperature. If that's the problem then you should consider a fault in water cooling system.