Got it chaps... if I turn it towards me it reduces the pressure, so snapping it back in the opposite direction should seal it. Trying it now. Will let you know.
Thank you both - very helpful.
Do I just twist the plastic red end to try and get it to seal? It feels a bit wobbly, not sure if I'm twisting it correctly.
First off... I know NOTHING about CH systems, I'd just like to be pointed in the right direction of a problem so that when I do call in an engineer I don't get the "Oooh that's a new boiler you'll be needing mate".
I opened the valve (at the back) as the pressure had dropped a little. Went...