done a quick google and came up with this, don't know if these are the same stones:

http://archive.oxfordmail.net/2007/7/19/170082.html

Since mid-June, the trustees have been carrying out a refurbishment programme. The work includes: the rebuilding of the wall and reinstatement of the banks of the leat running at the side of the land; replacement of the rotten palisade in the central area with sarsen stones;