In message , Mike Harrison
writes
What is it ? It's only attatched to the water circuit, so I assume it's
either a temp or pressure
relief valve.

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).

Should I investigate further - if something inside has broken/worn will
I be able to get a
replacement, or shall I just bodge up a proper cap to seal up the top
valve....?

It's an automatic air vent

BTW, posting photos to this newsgroup is not appreciated, please put it
on a website and post the link in future

--
geoff