I know that I'm twice as strong as an average person - equivalent to one of those rugger buggers, perhaps - yet I'd estimate that the prehistoric chipper must have been twice as strong again, just to be able to wield that hammerstone effectively!

I think you point out something crucial. Half a dozen farm workers can achieve far more than a whole load of urban types and keep at it all day. Yet most stonemoving re-enactments have involved the first type.
Add to that the fact you get gradually better and better at a job over the course of years and learn the absolutely most efficient way to do every bit of it. I reckon the efficiency of Gordon's two projects more than doubled between just the first time and the second, purely because minor adjustments were made, ones that no-one could have guessed on paper. Between Avebury and Stonehenge they'd have learned a lot more!

How long did it take you to become half decent at what you do?