The Stukeley sketches are strange. The rough sketch has one mark which is usualy a pond barrow and a another semi circle next to it but more on waden hill with its south part under the road. Later sketches show nothing but the final woodcut has a single ringed feature.
Its almost like he had seen something there but never really investigated it (maybe he didn't have the time as he was only here 2 weeks at a time.)
I really like the idea of it being the inspiration behind Silbury ;)
Maybe King Zil is under the little mound and SilDaddy
is to divert grave robbers' attention away from it :0)
PeteG