Pete,

Avebury Down Circle/Tumulus: Is this it?

NATIONAL GRID REFERENCE(S): SU11427126
DESCRIPTION OF THE MONUMENT
The monument includes the `Penning barrow', a Bronze Age bell barrow 600m east
of Avebury Down Barn. The barrow survives as a visible earthwork, the mound
of which is c.16m in diameter and stands up to 1.1m high. The barrow has six
large sarsen stones located around it which form a peristalith or circle,
along with five smaller stones and numerous fragments around its summit.
Surrounding the barrow mound is a 3m-wide berm and, beyond this, a quarry
ditch from which material was obtained during the construction of the
monument. It is largely infilled but is visible as a slight depression c.4m
wide.

Description from http://www.magic.gov.uk/rsm/21747.pdf