A few weeks back Pete G posted a link to this:

http://www.uwhg.org.uk/reports/other_meetings/silbury/silbury.html

which is the newsletter of the English Heritage Research Department. I remembered it mentioning that "the earthworks that survive on the summit are difficult to interpret..." (page 5).

Not sure if there is anything else recorded elsewhere (I'm no expert on this subject).

The flat top is the same diameter as Stonehenge, which once stood on there.
It looked spiffing in those days. It was used as a pigpen as the original builders, the Baconite people, worshipped them.
The stones were subsequently removed and re-erected on Salisbury Plain, near to the Baconite feasting centre at Woodhenge. They found it easy as they had anti-gravity. If you look carefully you can see slight gaps between the lintels at Stonehenge, this being due to the fact the reconstructed version's diameter is 8 inches too big, the result of a failure of communication regarding the true length of a megalithic yard. The project manager paid the ultimate price and was buried at Amesbury in a cramped position, his grave being deliberately constructed 8 inches too short, a deliberate and brilliant ironic message to the future by the chief Baconmeister.
Henge in early English means pigpen. Hence, it's original name, Stane-pigpen. Archaeologists have supressed the Truth.

Two theories I like, one is that Silbury is just a great spoil heap, dug out over the centuries to provide water - Loethar Respondek is the person who wrote the book on that one .... the other, 19th century, is that it was built like a pyramid, " that if you placed a pole on top of the hill the shadow would fall north on the level meadows, the daily gauge being about four feet “or almost, exactly that of the Great Pyramid”.
Think it was one of the earlier excavations, that a 'piece of wood' was found going down the centre, so you never know.....

A sign stating 'DANGER, STEEP DROP'

Don't know, but Peter Gabriel reckons you can see the city light if you climb on top.

Or something.

when i took my dad there, he said it looked like a massive slag heap and walked off. excellent i thought.

i then took him to avebury and he moaned that his knees were hurting.