In Geofrey Higgins 1826 book The Celtic Druid there is a drawing of a plan of Abiri and parts adjacent which clearly shows Silbaby with the words "Barrow cut through by the Romans.
While I'm intrigued by the idea that the Great Bank is a Silbury prototype I remain to be convinced - I'd also like to see an illustration of Higgins' drawing of 1826.
Topographical sketches of North Wiltshire by John Britton, 1826. p306 "This shows Silbury hill was ancienter than the Roman road. The makers of the same Roman road cut through another barrow, of smaller dimensions, east of Silbury hill."
The Great Bank is southwest of Silbury - John Britton must be referring to something else. 