Pet @ www.gymratz.co.uk ) gurgled happily, sounding
much like they were saying :

We've got a strange pipe lurking under the sink. It's closed off with
a valve but - as these things do - it's not exactly been behaving
itself as it should, and it's spent a few years having a little leak
whenever it thinks you're not looking.

From where is it leaking?

It's leaking from the "open" end of the valve. More of a weep than a leak -
at the moment, it's bone dry.

and what type of valve is it?

It's a valve. Looks like a bigger version of your standard ball valve.
Difficult to tell exactly, because it's not very accessible
/understatement