Is there any thread showing under the nut?
I wonder if it's a pattern part and the threads aren't quite long enough?
I had trouble with a generic drive shaft last week, ended up "machining" some metal off with my grinder.
Anyway, there are two choices, if the thread is too short, stick a washer on before you put the nut on.
If the nut is spinning, I usually put the jack under the hub and crank it up a few inches to put more weight on it.
As for spanner breakage, I am assuming they were generic "not very expensive" type spanners???
Try a Halford's Premium. They come with a lifetime warranty (at least they did....) so are a bit better made, lower carbon content.
If you were in the Inverness area, I could help, but since I can't see the problem.....
Nylock nuts go tight, but not "that" tight. It isn't cross threaded is it?