According to Prof Atkinson, writing about his Silbury investigations in Antiquity...
http://216.239.59.104/search?q=cache:Q6fkQfLF7hYJ:www.antiquity.ac.uk/Ant/044/0304/Ant0440304.pdf+revetments+silbury&hl=en&ct=clnk&cd=8&ie=UTF-8
"From undisturbed contexts on the top of the mound there came a sherd of flint-gritted Windmill Hill ware and a fragment of rock apparently identical with one of the varieties of Stonehenge bluestone (volcanic ash). Since it is now clear that the top section of the mound above the terrace is not a later addition, the deposition of this fragment ought not to be significantly later than the radiocarbon date of 2145 BC & 95 obtained for the building of the primary mound (Atkinson, 1969). The infer- ence to be drawn is that at least some of the bluestones were already in Wiltshire some centuries before their first use at Stonehenge itself."
(But .... isn't there also just a bit of a chance that the Stonehenge bluestone circle was first erected in Avebury and was then moved to Stonehenge?...a tenth of the effort subsequently used on the sarsens. Or was he wrong, was this Bluestone not bluestone? Did it go missing?)