The Wiltshire Gazette & Herald today reports David Dawson as saying,

“It tells us that in one of its earliest phases some kind of totem pole was erected on the mound, then subsequent additions to build the hill up were piled up around that timber.”

More here -
http://www.gazetteandherald.co.uk/news/4884791.Long_lost_theory_on_Silbury_Hill_is_uncovered/?ref=rss