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).
Nigel, I haven't read the above link very carefully as it's rather long, but is Atkinson saying that he found a fragment of bluestone while excavating the top of Silbury? Does he say how big the fragment was? 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).
