Check whether there's any residual pressure in the expansion tank first thing in the morning before starting the engine.

Edited 30 May, 20179 yr by Wino

Did you get this sorted Gareth?

 

Given how many things have been changed and the fault persists, I agree with Sepulchrave, it certainly sounds like an un-metered air leak. Do you know if the Intake manifold has been off at all recently? Might be worth checking the intake manifold gasket. 

 

I guess the Direction Wino is going in, is a Cylinder to Coolant or Oil Head gasket leak. I would do a Compression Test across all 4 cylinders and compare the readings. If Cylinder 2 is low, then perhaps a sniff test on the coolant tank.