Look what i've bought!

[Falcon 269]

'Hanging' idle is a symptom of an air intake leak between carbs and cylinder. 

Clean outside is not indicative of clean inside, especially pilot jets and airways.  Most likely cause of rubbish tickover.

Plug, plug lead or plug cap for misfire but check for spark first.  If spark is good on that cylinder, suspect carb blockages again.

Remind me, how far are you from the Clyde ... ?