Just to update, there was some air left in the radiator after I had fitted this auto bleed valve. The next day the air had auto-purged and after topping up the expansion tank it hasn't moved since.
This vent has been in place for 10 days now.
This shows the auto vent works, allowing air out but not coolant.
They are a very clever peice of kit, the internal seal will let air past and seal tight as soon as liquid comes. they will not suck air back in either as the system cools down and the pressure reduces (as in Combi boiler systems) I fitted two of theses Auto vents on Ma-in- law's heating system on to two radiators that were collecting air daily and completely solved the problem, because they auto vent. ~ i have an new OEM rad on my F and have a Mike Satur blled nipple to fit, might change my mind and fit the" AIKETGATE Patented Auto Vent" a great idea