if you over fill the header tank, it will piss out when hot

its an expansion system IIRC, so when hot the water level rises up

maybe it pissed out when hot, and when you switched it off etc... it will get hot, they all do, thanks to the turbo being part of the cooling system route...

have you put an 82 degree stat in it yet ? and 4x4 fan switch ? to help in the short term, then get an alloy rad, and turbo cooler, i.e. like the pro alloy one