Does anyone know why an EA will pump out water through the overflow hose about 10 mins after the engine is switched off?
With the old radiator cap it doesn't overflow when the car is switched off but if you put on a brand new radiator cap, it will overflow. I suspect in some way perhaps the old radiator cap wasn't sealing properly and maybe wasn't pressurising the system as much as a brand new cap does but doesn't that imply that something else is casuing the water to overflow?

Is the water temp gauge reading normal ???

I'm suspecting a head gasket issue - blown around a fire ring - the combustion gasses find their way into the cooling system.

If the good cap releases pressure when undone, when the engine has cooled down, then that's likely.
Best way though to prove/disprove this is to run a sniffer to check the coolant for gasses.

head gasket

So even with the cap left on after the engine is switched off, you're thinking the head gasket is the cause of it overflowing?
I know the head gasket was replaced recently but I guess it's a possibility.
If it is the gasket, what are the series of events that happen that causes the water to overflow 10 - 20 mins after the engine is switched off ~ even with the cap left on?