Well, I eventually found a '
jam nut' to use on the wheel hub that I am trying to take to pieces for servicing...
Unfortunately, the final score was:
Pathetically Wimpy Jam Nut 0 -
Tightest Nut On Tight Street, Tight City, Tight Nation, Planet Tight 17!
The jam nut couldn't jam tight enough, so that side kept coming undone rather than the nut on the other end of the axle, which was the one I need to shift. I don't know whether corrosion is the problem, or whether somebody cross-threaded the reluctant nut.
I could try finding a beefier jam nut so I can pinch it up even tighter, but for now I have squirted GT85 on the stuck nut to see if any can get into the thread overnight and loosen it. I think I will also try immersing the nut in boiling water before my next attempt to see if heat helps. (I don't really want to apply a flame because I might damage the nearby freehub. And set fire to the GT85, of course!
)