A tunnel! Now why didn't I think of that? Maybe we could run it under Silbury itself thereby providing an in-depth (no pun intended) archaeological examination of the Hill while at the same time reinforcing its foundations and sucking out the polystyrene in-fill from below (simultaneously of course pouring in a chalk compound from above). The Avebury Bypass Tunnel could even be enlarged BELOW Silbury to provide an underground car park for Avebury visitors or, perhaps, even to emerge a little further west at West Kennet Long Barrow! Perhaps WKLB could even be set at the centre of a super roundabout - visible to all but accessible only to the foolhardy.

The tunnel possiblties are endless! Thank you (and I haven't even begun to consider the flyover option yet) though I still kinda think a simple bypass is the answer.