Success! I decided that I should get something that could inflict a little more impact. So, my mini kango type sds drill was pressed into action with a pointed bit and voila! said nut is now off!
Inspecting the nut, the threads are not very good and measured different pitches around the the thread - this probably added to the 'tightness'. I tried a spare nut and that is a much better fit.
Now to make my M18 x 1.5 bolt to take the alternator bit off......
Thanks for any input - it got me to think differently.