Hi Paul, has a blown head gasket or worst, a cracked head been discounted yet?
You have probably already tested for this and this is just my two cents worth. Some of our local radiator shops has some sort of attachment that is put on top of the radiator filler cap receptacle (cap removed before starting) once the engine has reached operating temperature. The mixture inside the attachment then reacts with any CO2 in the water system, indicated by a colour change. The CO2 in the water, if any can only come from the combustion chambers, thus indicating a gas passage somewhere in the engine. So, if no colour change, you dont have a problem in that area.
Regards.