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.
 

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