Thanks for all the replies guys. Engine has just been down to the mechanic who also had the carbs off, he also could find nothing wrong with them. He seems to think maybe the top crankcase seal is a bit worn, sucking in air intermittently. Is this a possibility?
He's re tuned it, I've just been for a run in it for about half an hour with 10 mins of idling, done a good job albeit the cough is still there when I run it on muffs at home. It only coughed once. it's much better than it was, but the underlying problem is still there.