Mike Dodd wrote :
So, my question is (and I know there's not much to go on) - is there any
likely "gotcha" on a closed system where I can look for a leak or anything?;
I'm not competent to repair anything, but if I can spend my time finding the
puddle of water, maybe I can help save her some cash when she needs to get an
engineer / plumber in.

They can be very difficult to find....

It doesn't take the loss of much actual water to cause the pressure to
fall.

One way is to drain the system and pressurise the system with air. Air
can escape more easily than water, so you might be able to hear the air
blowing out of the spot where the leak is.

--
Regards,
Harry (M1BYT) (L)
http://www.ukradioamateur.co.uk