As I am on the sceptical side of a 'carousel' Stonehenge for grinding wheat, but it did make me look at bronze age looms for weaving the sails, which are vertical and roughly a metre wide, now they seem to have rollers for winding on (for length) but surely width would have been important? As for flax being used, it does need an awful lot of work to make it flexible for the weaving - retting and scutching which takes weeks.
Historically the mill seems to have been introduced in saxon times, querns were forbidden, you had to take your grain down to the mill so that a 'tax/portion' could be taken....