An account from 'An Illustration of Stonehenge and Abury' by Henry Browne, 1823 (in Wilts Arch & Nat Hist Mag v95, 2002). It's an eyewitness account of the 1776 dig.

"... This elderly gentleman [a Mr Hickley from Avebury] when a youth, was at Silbury Hill on the occasion of some miners sinking a large hole or well down the centre of it to the ground on which it began to be raised. In doing this they found a piece of timber continued down the whole way, evidently for a centre from whence to take the measurement of the hill in working it upwards."

http://www.themodernantiquarian.com/site/30/silbury_hill.html#fieldnotes

An account in The Gloucester Journal on 9 November 1736 describes how a dinner was served on the summit while between 4000 and 5000 people sat at the foot of Silbury and on a facing eminence, all of which was made a very agreeable appearance. A bull was baited at the top and bottom of the hill and: There was also backsword, wrestling, bowling and dancing. The same diversions were repeated on the 2nd day, and also running round the hill for a petticoat. The 3rd day the bull was divided by Mr Smith amongst his poor neighbours on top of the hill, where they diverted themselves with bonfires, ale and roast beef for several hours...