The stubborn exhaust nut is finally off.
I tried, releasing fluid, a special spanner, a blowtorch, but finally it got down to brute force and ignorance.
A rummage in dad's old toolchest revealed a couple of old heavy caulking tools,similar to heavy blunt chisels and with a glove under it to protect the alloy in case I slipped I then battered the nut with the hammer and chisel combo until it was off.
The nut is now junk, I did try the other nut in the port working it back and forwards to try and clean up the threads in the alloy cylinder head and it has made a visible difference in the definition of the threads in the port, I revisit this sometime later when its all going back together .

Here are the old nuts the stiff one is the one on the right.

New castle nuts are now on the shopping list.
From here:-
http://www.morinispecial.it/joomla/index.php?option=com_wrapper&view=wrapper&Itemid=168

I also tried a quick wipe over of the wiring with a paraffin soaked rag and it was like moving from B&W movies to technicolour is one wipe, gone is years of sticky grime. I'll do it more thoroughly in the next few days.