http://www.themodernantiquarian.com/post/33314

Looking at the shot in the link above - the bank reinforces the A4. The road goes along the spur of Waden Hill, so the 'great bank' is there to shore up the road (did the Romans do that kind of thing?)... if the mound had predated the road, surely it would have been used to help create the bank, and thus destroyed? I'd have thought the mound came after the bank, and is therefore, at the earliest, post-Roman.