So spent more time on it yesterday, the carb has certainly made things a lot better and advancing the ignition almost cured it.
Went back to my first instinct and started swapping plugs out again.
Yep, traced the last bit of misfire back to a failing spark plug. Swapped the lot to be sure but that was the last piece of the puzzle.
Could be I just moved the fault from 1 cylinder to another last time or maybe a 2nd set of dodgy plugs.
Still lots of nice shiny bits in the engine bay now.
Many thanks for all the comments folks.

Great news and well done on your persistence! often it's the simplest things.