Thanks for the link to the map, Grendel.
Unfortunately, it times out, so you have
to go to:

http://www.ca-mapping.co.uk/maile_r3/Default.htm

and type in the place name.

I had a look, expecting to see large areas
of downland designated. All I saw around
Avebury was Silbury Hill(!) and parts of
Fyfield, Knoll and Cow Downs coming under
the scheme.

baza