Hello,
I've never read Devereux, but generally I would beware of 'comparative' theorising both religious and historical. These ideas are popular because they offer simple explanations of the many diverse mysteries left in such confusing abundance by ancient and vanished peoples. Really, though, to try and construct specific links between Silbury and the Pyramids is just distracting from the wonder of Silbury itself. There's also a long and dodgy tradition of re-writing history to fit people's prejudices, i.e. that no-one in ancient Britain could have been together enough to build Silbury without 'outside help'. Finally, any society anywhere in the world trying to build something really big would find the most stable structure starts big at the bottom and gets smaller the higher it gets, like Silbury, the Pyramids, Palenque temples, Ziggurat of Ur etc, also sandcastles...