replaced by the Operations staff wooden offices. The scene was now set for the wartime era, which is, in a national context, considered to be the most interesting and significant part of the Priory's history, and that of the RAF. In January 1939, work started on the underground Operations Block which was occupied and commenced operations on 9 March 1940. The average depth of the excavations was . In the Spring of 1940 Air Officer Commanding RAF Fighter Command, Air Chief Marshal Hugh Dowding, by then a widower, lived a very quiet domestic life with his sister Hilda at a house called Montrose (no longer standing) in Stanmore. He rarely went out socially and with an immense work load was happy to forgo all but duty functions. Every morning he would walk through the grounds to his office in the Priory. Dowding would often meet General Frederick Alfred Pile, who was the General Officer Commanding Anti-Aircraft Command, who was walking from his home, Glenthorn. The pair, who were firm friends, would use the time to talk about the war and other subjects. Dowding left visits to other stations in Fighter Command to his Staff Officers as he was much occupied with the work in his Priory office. However, he did pay regular night-time visits to research establishments and the air defences around London when The Blitz started in earnest in September 1940. These visits often followed periods of up to 48 hours at his desk leaving Dowding exhausted. Lord Beaverbrook and Winston Churchill made Dowding unwillingly relinquish command of Fighter Command on 24 November 1940 after he only approved some recommendations in a report to improve night air defences created by a committee of enquiry chaired by Sir John Salmond. Dowding's time leading Fighter Command left a lasting legacy on the organisation. His creation of the Operations and Filter Rooms, the essential elements of the Command, Control and Communications system, became the cornerstone of the Air Defence System. The system he developed gave air controllers the best chance of scrambling fighter squadrons to intercept Luftwaffe raids before they reached their targets. This rapid, flexible approach was essential because there were insufficient aircraft and crews to keep fighter patrols continuously airborne in 1940. He also played a significant part in encouraging research into night-fighter equipment and tactics. Historian and Dowding biographer, Robert Wright, who served as Dowding's personal assistant in the