"Sine qua non" ("Indispensable")
History of 540 Squadron:
No.540 Squadron was formed on 19 October 1942 at Leuchars from H and L Flights of the Photographic Reconnaissance Unit. The squadron's Mosquitoes carried out reconnaissance missions over Norway and based a detachment at Benson for similar flights over France and Italy. From Leuchars, long-range trips were made to German and Polish Baltic ports, while another detachment covered southern France and Algeria from Gibraltar in preparation for the landings in North Africa. From February 1944, the squadron was wholly based at Benson and sorties ranged as far afield as Austria and the Canary Islands. At the end of March 1945, No.540 moved to France for the rest of the war, returning in November to Benson where it was disbanded on 30 September 1946.
On 1 December 1947, No.540 Squadron reformed at Benson, and used its Mosquitoes for photographic reconnaissance and survey duties, converting to Canberras at the end of 1952. In March 1953 the squadron moved to Wyton, where it was disbanded on 31 March 1956.