Newbie wonders - "How, oh how?"

Large pair of grips on the valve, spanner on the nut.

In needs to be tight, but not strangled.

It happens that GPG formulated :

A spanner, self-grip, or even Stilsons placed on the valve, whilst another spanner is used on the connection. If necessary, put some rag under first to protect the finish. Pull on one and push on the other.

They need be no tighter than what is needed to provide a seal. Just nip them up gently, then a bit more if their is a leak.

Thanks for this advice. Looks as though the joints with olives are damaged because, the more I tighten them, the worse the seepage becomes. Not exactly leaks - I have to run a very dry finger along the top and bottom interfaces to find that they are damp. But I guess that that is not satisfactory. Back to the drawing board? I think I got an exagerated idea of how tight was tight because it was pretty hard to undo some of them in the first place - PLUS, I always believed that the length of a spanner was designed such that the average person would not be far out if he/she used reasonable force when tightening. I wonder if anybody has used a torque wrench in any of these plumbing situations and recorded them so that we could benefit from such data?

This is roughly true of high tensile nuts and bolts.

FWIW, I'd take the fitting apart and clean it all with wire wool, then apply some Fernox silicone sealant which you'll get from a PM. Wonderful stuff.

It's possible you've got a deep scratch on the pipe running under the olive - or some damage on the tapered faces of the nuts. Either way I'd bet the Fernox will sort it.