Hang on a mo... this might make me very, very unpopular but does anyone actually know WHY English Heritage chose polystyrene to support Silbury's vertical shaft?

Polystyrene sounds like a ghastly and unsympathetic material to use at a site so important to so many of us but it's actually kinda intersting stuff. A sheet about an inch thick and some 4x8 feet in size can be balanced on the tip of your finger (in other words a material that can be easily airlifted to the top of Silbury).

Sandwiched between two sheets of acid-free board polystyrene is still incredibly light and so rigid you can't bend it. This polystyrene/acid-free sandwich used to go under the name of Gatafoam and was/is used by conservators in some of our major museums and art galleries as a mounting material for two-dimensional art.

Could be that the polystyrene fill at Silbury is the most appropriate solution in the short term. Once Silbury is stabilized and a more sympathetic fill decided upon the polystyrene is going to be a hell of a lot easier to remove than X tonnes of some other material.

Think about it - a few years back someone would have authorized reinforced concrete (and try getting that big baby out of the centre of Silbury).