KA sport overheating! How to test heater control valve?

[KA Kev]

Hi all, first post..

After reading some of the posts about KA overheating probs it seems that a lot of the times its down to the "heater control valve" .

Is there anyway I can test to confirm before I replace it?
Bypass? Block off? etc..

Symptoms:

Heater works ok.. Pipes hot..
Overheats but only after several miles..
Rad fan kicks in BUT rad doesnt feel hot ! just warm..
Expansion tank cold/warm..
No pressure in system..
No water loss except if you ignore temp then just boils over..

There is no pressure in the system even when the overheat light comes on.. I dont understand how there is no pressure even when normal temp could it be expansion tank cap?

Pressure tested and all ok..

My thinking was initially thermostat but can see a lot of heater control valve related overheats..

Opinions and test suggestions very welcome.. TIA Kev..