John Murrell
There is no real evidence this was a military building - it has what were once glazed windows to the South and appears to have had trees to the South obscuring the view since before WW2. Various pieces of evidence including the type of fencing round the site and the fact the similar but smaller building on the North Side of the North Downs Way (missing from OSM) was still in use in the 1950's makes me suspect this was operated by the Sutton District Water Company.