Thanks for your comments.
Glad to know I've not picked up a duff engine.
I agree it's not really worth worrying about, but I think I wasn't expecting any vibration at all. I think I do too much pottering, as I agree that it smooths out. That's perhaps why not noticed on the 6hp, as it probably got more of a workout. I guess I was thinking bigger = smoother, but it sounds as though we need to get to 3 or more cylinders before that becomes true ? Anyhow, all fine - it's basically a lovely engine
Not sure what pitch my prop is - maybe I can measure this ? Will then just generally monitor for a bit, as too much pottering again means no conclusive findings yet ! but looking like... occasional ventilation - and when fairly well loaded.