Hi Andy...The thread on the sleeve is hardened so will survive some abuse. Before fitting another nut give it a good clean up on a rotary wire brush. When you get a replacement nut if it is not totally free on the thread you can usually 'work it in' with a little fine grinding paste.
The original tool for compressing the spring is pretty poor..if you use one of these it is quite easy to get the nut cross threaded at the start. If the nut even begins to go tight when you try to do it up immediately back off the nut and try again.
It is worth looking on the 'Tech Section'of the forum to see Hans's solution to this problem...(the clutch can be assembled off the bike).
I made up a non standard tool for my own use which will work with the clutch still fitted to the bike but I haven't produced a drawing of it yet....Ian
And here is the solution to compressing the spring on the bike. Similar to Ian's purpose made tool, this one was made by my friend John, using an old piston a piece of scrap ally and some 12mm studding. Ron