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The genteel walkers across the National Trust fields seem to follow the Hippy trails of the festivals. (Though some of them may well be the same people.)
I have a copy of Alternative England and Wales from 1975 (https://www.youtube.com/watch?v=FMSOq9ZTRj8 for a guided tour of its contents) and Stonehenge isn't mentioned, and the guidebooks all are of Avebury and Stonehenge.
Much of the present iconic status of the monument can be traced back to the Festivals and not only should there be a proper research project into them but they would also make a fantastic exhibition subject.
Mike Pitts has written about the festivals here - https://mikepitts.wordpress.com/2013/08/29/where-was-the-stonehenge-festival/