Hi folks,
Thanks for the useful replies.
The kind I am looking for is the kind I have seen in the past. They are very simple non-moving parts, it is a screw-in thing that resembles a greese nipple, but it has a disc approx 3-5mm in diameter which is either copper or a tin-foil looking substance. I imagine they are about 50p each and I like the idea of them because, unlike some mechanical relief valves, I don't see how it could fail to work.
I thought these things were as common as muck, but maybe I'm wrong.
Having had a good look around all I can seem to find are mechanical valves. I'm sure these are pretty reliable but I would the type above! Just need to locate a supplier.
Ultimately I guess it won't make a lot of difference though.
Rich