Thoughts on Silbury’s innards:
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.”
It was nearly 50 years after the event, so although you might expect some elaboration / misremembering – surely this isn’t an obvious yarn to tell? Wouldn’t you be more likely to come up with the old ‘skeleton / treasure’ option rather than a central timber?
Another contemporaryish account is interesting: James Douglas, in his 1793 ‘Nenia Britannica’ recorded that the Duke of Northumberland’s foreman of the work (a Colonal Drax) “had a fancy that this hill had been raised over a Druid oak, and he thought the remains of it were discovered in the excavation.”
And indeed, Richard Atkinson, leader of the 1960s BBCized dig thought that the 1776 shaft would probably have destroyed any central deposit at the base of the mound, and no doubt anything vertical in the core...