So I did have a leak in the actual line from the valve block to the rear right bag.
I bought some rather fetching bright yellow PA12 nylon airline from
here and some plastic conduit and with a borrowed lift, ran new lines for the rear right bag, and the tank down the chasis rail, accros the rear cross member and connected up.
Getting the line across is easy, getting it in to the clips is a pain in the neck. It's possible with help, swearing, long screwdrivers, short screwdriver, prongs, hooks and selotape. I also encased the line in split conduit where it passes by the exhaust and tyres to try and stop any future holes developing.
No more overnight sag, so that seems to be a win.