Wow the grex valve i saw was on american amazon for $29. But says cannot ship internationally.
On amazon uk they are charging £63.75 + £3.75 delivery
There were two other alternatives an iwata fixed valve for £23 ish and a gothobby quick connect valve for £31 ish.
I,ll check machine mart im sure they will be cheaper.
Try Lion Art here in holland http://www.lion-art.nl/contents/en-uk/d45.html