[WAH]

Thanks guys, been tied up on domestic trivia,
Utopia. Much appreciate your kind words, having inspected the nut a little more thoroughly I'm inclined to believe I was mistaken in my earlier ramblings, I don't believe the nut/bolt assy had moved at all, in spite of all my efforts, suspect it was in this situ when I started on it.
As for drilling the head probably worth mentioning it's going to be a high tensile item.
Regret I don't have air tools of any description, have to see what mate brings over to play with.
Edit.
regarding mangled swingarm axle bolt head (adj sprocket) the incorrect sprocket fitment won't have improved this mess! Can see no evidence of chain attacking crankcases tho, so assume damage is down to previous lack of chain adjustment.