Julian Cope presents Head Heritage

Don't forget to bear in mind that Silbury was thought to have been modified at the beginning of the 11th century.

Jim Leary and David Field write in The Story of Silbury Hill

... Silbury itself seems to have been left alone for the few centuries after the Roman rule, and it is not until the beginning of the 11th century that once again it becomes the focus of attention. For in the 11the century it appears to have been modified, it huge size perhaps providing a convenient lookout point to the old but still serviceable Roman road.

Evidence of post holes from this period had been found on the two uppermost terraces of Silbury (though not thought to be part a defensive palisade - simply a way of getting to the top).

I am persuaded by Sanctuary's theory that Silbury was built where it is because of the watery connection (given that even today the land between Silbury and the Winterbourne is often under water).