I think I might have just identified my problem. The original owner was insistent that the bike used to run beautifully with the current jetting so I started wondering whether there might be another problem.
It has a new piston, the dents blown out of the exhaust and silencer repacked so that was all ruled out.
I got a bit more analytical about how the bike was running - bogging at low revs but clearing higher up, regardless of throttle opening, suggesting it could be something other than jetting.
I had an idea, did a bit of googling which supported it, then pulled the reeds. Bingo! 2 of the petals are chipped along the edge - just enough to affect the seal and allow a little leakage back to the carb at low air velocity.
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