In article , Mike Harrison
writes
The valve had a plastic cap (like a car tyre cap), and two O rings at the bottom
which have gone
hard and cracked.
The plastic cap doesn't look like it's meant to take any pressure, so I suspect that
whatever's
inside is leaking and the cap had been holding it until the O rings cracked.

Unfortunately it's buried behind pipes so I've not been able to unscrew the top bit
(which I assume
does unscrew).

It's an automatic air vent which has a float and valve assembly inside. The
valve commonly fails over time making it leak. The cap can be screwed
down to remove that risk, but if you get air into your system is may collect
in the boiler and risk damage. As a short term measure you can screw
down a new cap, but I'd replace it when you get a free moment (before
winter). They're under a fiver from a plumbers' merchant.

BTW, I know your pic was small, but this is meant to be a no binaries
group, the proper way to show one is to upload it to a free pic site & post a
link. Thin end of the wedge, blah, blah, yawn.

HTH
--
fred