"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."

Well, I'm not sure. The bank is huge and long, and is a solid chalk face right at the bottom so I'm wondering if it's just naturally eroded by the river over aeons. Or maybe half and half, it's hard to say.

One thing struck me though: where did the material for the mound come from? There's a Silbury-like ditch/quarry on East side, but that's only about a twentieth big enough. And there's a Silbury-like (again! but I'm not claiming anything other than coincidence) square pond on the West side, and some could have come from there. But not enough.
So I can only assume it came from the bank.

(As to this, the pond is very square and apparently artificial, and one of it's straight sides is the bank. So maybe...? In addition, the river that joins the pond is dead straight, and also hard up against the bank. This will all look stupid when it's found to be nothing, but the mound, the pond and the river all look like a bit of town planning to me).