Leaking rad bleed screw, does anyone recognise make or type please

Hi all
Just been doing the pre winter checks and went round bleeding the rads. A couple of the bleed screws in my set up are dodgy, whereby they do up to a certain tightness then slip off loose again, one of which is now dripping when the heating is on.
I haven't removed or checked the valves as I'm wary of making them worse, I was hoping I could just buy some replacements. Unfortunately there's nothing similar on eBay or at the merchants websites. House was built around 2002.
Any help would be much appreciated.
Cheers Chris

change the full unit ,standard size ,seal included on most of them

As above straight swap, the plastic bit is not needed it is just to direct the stream of water when venting.

http://www.screwfix.com/p/radiator-vent-blanking-plug-chrome-bsp/40972

Nice one gordongas and footprints, exactly what I want. I scoured screwfix but never saw these.
Much appreciated
Chris