Newbie 2.2 dti coolant pressure problem Help!!

[bigred1983]

I have a pressure problem, the water system has very high pressure, if i drive the car to temp and leave it overnight by the morn it still has pressure in the expansion tank. There are no other signs of head gasket failure and a sniff test came back negative, ive replaced thermostat which opens at temp and new water pump and new rad but im loosing the plot now!!!

Any ideas

Thanks people