I've reached out to Coolex, I think you're right it's 8mm which matches the header tank's small inlet and it's position suggests it's there to vent air in the rad. However my current setup doesn't have one, and the header tanks small inlet is connected to the inlet manifold which I've since read has a Jiggle Valve (unless mines been removed which I can't tell without pulling the inlet off I guess) which is designed to capture air bubbles moving around the system and feed back to the header tank. So feels like I can probably just blank off the 8mm outlet on the rad. https://andrewrevill.co.uk/JiggleValveReplacement.htm