Nigel wait
In 1776 Colonel Drax and the Duke of Northumberland dug down from the top. Accounts tell us only that they found a small sliver of oak and ‘a man’, presumably a skeleton, at the base of the shaft, though a newly discovered report says the hill consisted ‘of chalk and gravel thrown together by the hands of men’ and that ‘there were many cavities in it’.
from http://www.britarch.ac.uk/BA/ba80/feat1.shtml
If the account is to be believed, there was at least one burial, apparently a simple one, but on center. It was excavated in the Eighteenth Century by treasure-hunting nobility. If they didn't leave the body where found, it could have wound up in the spoil, or (at a stretch but not unreasonably) have been reinterred to the Duke's own cemetery (if any) or that (again) of the Colonel's regiment. In any case, we may have this simple fellow to thank for their not digging clear to the bottom and destroying what was later encountered by Merewether, (& Atkinson after him). The mention of gravel in the later report seems to indicate they got through Silburies 3 & 2 ... the 'man' being atop or in the outer layers of Silbury 1,
(close call)
For those interested, the thread "Silbury Hill, why build it?"
http://www.megalithic.co.uk/modules.php?op=modload&name=Forum&file=viewtopic&topic=565&forum=4&start=140
in Burnham's Megalithic has a lot of theorizing, (some mine), together with friendly discussion, evolving ideas, a referenced debate, all around gentlemanly conduct [;-)] and more
Also to be considered is that the monument might be better understood in the context of Greater Avebury; a topic explored in the thread "Silbury, Silbaby & the Environs"
http://www.stonepages.com/forum/index.php?showtopic=1114
on Stone Pages, (holy smokes, 15,548 views)
It seems possible to make educated guesses as to the purpose of the Hill ... but that 'final answer' is probably beyond reach
