In Jim Leary and David Field's "The story of Silbury Hill" The Flagstones enclosure is mentioned in relation to Silbury (in one of it's earlier phases) and Stonehenge.
the book ties together the early Silbury enclosure with the first phase of Stonehenge and the Flagstones which was said to be over 100m in diameter.
Rodney Castleden's book 'Neolithic Britain'* says that half this ritual enclosure still exists unexcavated - the rest was lost when the Dorchester bypass was built. He says: "The most striking thing about Flagstones has been the discovery of neolithic chalk engravings, carved in the near-vertical walls of the ditch." The enclosure was constructed on the same east-west ridge where Mount Pleasant and Maumbury Rings would later be sited.
I was in Dorchester's museum today, where you can visit one of the carvings.
*What you probably really want is PJ Woodward's 1988 Antiquity article about the discoveries, 'Pictures of the Neolithic'. I haven't seen that though.