All over the place really. On my first summer solstice visit in 1971 it was little more than a gaggle of people sleeping in the ditch around the stones. These two photos from Julian Richards’ excellent collection published by English Heritage show where it went from there: first immediately outside the earthwork to the south-west, so that it faced the rising sun on June 21; then across the road into what we call the Cursus field, National Trust land between Byway 12 and the Fargo Plantation. At its greatest extent it did run onto the Cursus itself (thanks for the comment, Francis Stoner), and spread eastwards into the Avenue field. Damage was reported at the time to the woodland at Fargo and to some of the barrows.
It’s high time a proper academic study was done of this story. There must be a great deal of information out there in the form of press reports, memories, correspondence and snapshots (and English Heritage, National Trust and police files), but it needs to be collated and assessed before we ever get a real picture of what happened in all its details. Would make a good PhD.