Or, Steve, couldn't we impose the downward force on the tensioned rope using Type 2 levers?
i.e. you'd have a fulcrum (the end of the lever wedged under an anchor stone), the load (a rope tying the tensioned rope to a point below fairly low down on the lever) and then you'd just pull the lever down.
The sarsen would come up a bit, you'd wedge it into it's new place, re-tension the rope and repeat.