In 1980, Duncan wrote "A Christmas party for the moles" for the New Statesman -  a report based on his investigation and journey through London's secret underground tunnel system.  Maps of the tunnels were included in his book, War Plan UK, and are provided below. 



Planning for a deep network of Whitehall tunnels and War Rooms began before the Second World War.  Each service ministry. the Cabinet War HQ and other civil and military administrators had protected accommodation in the 'citadels'.  A deep level tunnel and lift shafts linked each citadel. From 1950-53, new bunkers were built under Whitehall. - From War Plan UK



The Post Office (now BT) network of deep level cable tunnels under central London. Most were built to give protection against A bombs during the 1950s - From War Plan UK



Similar underground tunnels were built in Birmingham and Manchester.  The above image is a map of the Manchester Guardian telephone exchange.