These early "Y" stations were formed into the Radio Security Service in 1940 with headquarters at large house named "Arkely View" near Barnet from which search instructions were given and results were collected and forwarded to Bletchley Park. In addition to the Post Office "Y" stations, a number of experienced radio amateurs were recruited who were allocated frequencies and times at which to monitor and record transmissions heard. Starting with about 150 in 1941 rising to 2000 by the end of the war.
Initially the main interest was illicit transmissions from within the UK and stations were set up to provide reliable detection of all areas of the UK. The hand drawn map on the right was located in an MI8c file at the National Archives and shows an early plan which included one station in the Southwest of Ireland but the declaration of war and Irelands neutrality prevented this station becoming a reality. However one was eventually built at Gilnahirk in Northern Ireland, see link below.
Like a number of other RSS stations, Gilnahirk continued operations after WW2 into the Cold War as did the Foreign Office Y station at Brora that was in use until 1986, see link below.
When no illicit transmissions were found within the UK in 1940* the attention of RSS turned to the large volumes of German Secret Service transmissions heard from neutral and occupied countries in mainland Europe. The RSS was first run by MI8c and MI5, but as more external traffic was being monitored it was taken over in autumn 1941 by MI6 who were responsible for intelligence matters outside the UK. From that time RSS was run from Hanslope Park by Ted Maltby under the direction of Richard Gambier-Parry.
MI6 took over day to day control of the "Y" stations from the Post Office and added extra D/F stations at Bridgwater (Stockland Bristol) and Wymondham. Voluntary Interceptors proved valuable since they were experienced in digging faint signals out of the background noise using inexpensive equipment. However, in the beginning, a meeting at The War Office in December 1938 shows that not everyone thought using them was a good idea.
*The main concern in 1939 was the use of wireless transmitters by German spies and the use of homing beacons to guide German bombers to their targets. A small number of "spy" transmitters were heard throughout the war but only a few knew that these were all from agents that had been captured and "turned" and their transmissions controlled by MI5.
Copies of handwritten documents at National Archives show the development of RSS.
N.B. The station marked "Woodstock Hill" above refers to Woodcock Hill, Sandridge, near St Albans. I believe this was the original P.O. experimental D/F site in a field adjacent to the Sandridge Foreign Office Intercept site.