Thanks for that - useful to know the proper names for these things.
just to make it clear for other folks like me who don't know what we're talking about - bleeding from the blanking valve (big hex one) definitely had an effect on mine, when bleeding from the bleed valve (little square one)...
it's possibly pretty simple
it seems there are two points you can bleed the rad - one little (square - and about 5 mil on mine) and the other is a larger one (of order of a couple of centimeters and hexagonal) which is part of the radiator itself and on mine situated at the top at one end...