Ammending the last post, I doubt if liquid water reached Silbury 1 after Silbury 3 was built
http://books.google.com/books?id=wQ4AAAAAQAAJ&pg=PA74&lpg=PA74&dq=water+absorbed+by+chalk&source=web&ots=pq-_hnEFGU&sig=qvcuBbV8X4QVmysgy0QIzJAxrCE
^ This link from Google books has chalk able to absorb as much as 2 gallons of water per cubic foot -- impressive

http://www.air-worldwide.com/_public/html/air_currentsitem.asp?ID=1298
^ This one from 'AIR', has some detailed if small maps of precipitation and drainage which make it look unlikely that such a deluge will occur as will saturate Silbury Hill to its base from above.

A look at the OS map has Silbury on a platform up to 9 meters above its moat and/or, (the map is somewhat ambiguous here), the level of the Kennett. This would provide a buffer during seasonal Gypsey flooding. In flood, I expect water would wash against the sides of the Hill. Here again, the chalk's ability to absorb water while slowing the progress inward would, imo, have prevented liquid from reaching the heart of the Hill.

In other words, Silbury Hill's chalk retained humidity around a center which water as such never reached. It underscores the necessity to preserving the hill and its contents of replacing chalk with same.