Furthermore, there's no explanation of how the polystyrene will be extracted - and we all know how that stuff behaves when it's broken up. Unless the top of the shaft is covered with a temporary structure (which would also help protect the interior of Silbury from rainfall) there's a strong possibility that pieces of polystyrene will get blown all over the surrounding countryside.

That's just the external problem - more worrying is that the interior of Silbury will be forever contaminated with pieces of polystyrene.