Update on results: first of all, good news. Engine is now a different animal - what a world of difference (and great relief for me) - it is well behaved.

Changes made:
a) leaned low-speed needle 1/8 turn
b) changed to OS 6 (hot) plug
c) fuel changed from store mixed to Cool Power (10% nitro both) - not by choice - store is no longer selling their own pre-mix.

I believe all three elements played a part in improving the engine's running characteristics.

Anyway, thanks again to all for their worthy input.

Cheers
Ken