"No Moon Tonight" by Don Charlwood offers some clues. Don was a Lancaster navigator with 103 Sqd based at Elsham Wolds. Aerodrome control callsign was "Hazel". Perhaps this may have changed on a regular basis to prevent any eavesdropper being able to identify the real location.
Aircraft PM-L calling Elsham Wolds aerodrome control - "Hello Hazel control L London"
Calling another agency - "Hello XXXX Hazel L London"