"English Heritage have had to temporarily halt the repair works at Silbury Hill because of
complications caused by instability above the central Atkinson chamber within the centre of
the Hill, which has also been exacerbated by the volume of rainfall in the last week or so.
Our expert archaeologists and engineers are developing a solution to take the
project forward, with the aim to stabilise the Hill for the long term. We will issue a new programme for the completion of the works as soon as possible."

http://www.english-heritage.org.uk/upload/pdf/Silbury_Hill_web_Update_Week_10_Delay_statement_-_new_version.pdf