The header tank/expansion tank is the same thing if it plumbed into the main system - you essentially mount it as the highest point of the cooling system and half fill it to allow for the air to be compressed inside, it helps if you feed a small coolant pipe into it as well so the water does not become stangent.
Another option is the way you have it plumbed up at the moment (partly), the overflow pipe next to the filler on the rad should be vented to either air or the bottom of a catch tank so if a vacuum is formed inside the cooling system once it has cooled down the water gets sucked back into the system instead of air being drawn in. If you use this method the un-pressurized expansion tank should be vented at the top to free air.
hth
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