Good news all around.....
Stripped the top end down and replaced the base gasket, cleaned the piston and rings (might as well)
Installed new reeds and new inlet rubber and then started her up, sounds sweet again! nice tick over and started practically first kick.
No lean runaway (although won't be able to confirm that until she's hot again when I go to work tomorrow but no reason to doubt It hasn't been solved)
Then I proceeded to install a replacement original exhaust system courtesy of some chap i met at the petrol station.
See this post for details
And now it sounds amazingly quiet!!...... boy did I have a noisy end can before!!
Now its all done... I'm going to have a lay down in the sunshine and top up my tan as I doubt this weather will last much longer!