Celia Fiennes wrote travel books in the 18th century and her take on the stones was that they were ‘a warning for boggy ground’, an interesting if elaborate idea(?!). I have my suspicions whether she actually visited the site, as she talks about Long Meg as being in the middle of the circle:
A mile from Peroth in a Low bottom and moorish place stands Mag and her sisters; the story is that these soliciting her to an Unlawfull Love by an Enchantment are turned wth her into stone; the stone in the middle wch is Call’d Mag is much bigger and have some fforme Like a statue or ffigure of a body, but the Rest are but soe many Cragg stones.
...they affirme they Cannot be Counted twice alike as is the story of Stonidge [Stonehenge].
quoted by Grinsell in ‘Ancient Burial Mounds of England’ 1936 (but I will look at the original soon).