I'm trying to bleed a new radiator for the first time and can't work out how the bleeding valve works - it's a different sort from usual. It's round, with a slotted metal bit in the middle which a screwdriver fits inside but won't turn. Around it is a circular plastic collar which turns and has what looks like a tiny nozzle on one side, but doesn't come off. A normal bleeding key doesn't seem to fit anywhere. Can't work out what to do! Any ideas?