In Biggles Hunts Big Game, Delmar was a fictional R.A.F. station in Hertfordshire. At the time of the events in the book, the station had been closed down and only a small care and maintenance party was left in charge, headed by Squadron Leader Crane.
After Biggles had been assigned the case to investigate the international counterfeiting gang, he very soon discovered that the opposition had found out where he lived and had mounted a surveillance post outside his Mount Street residence. Moreover, John Robinson had paid a call on him and tried to bribe him to drop the case and, when that failed, tried to kill him with an explosive cigar. Biggles decided to move to operational quarters and asked Air Commodore Raymond to arrange for him and his Air Police staff to use Delmar. The R.A.F. had stopped using it but it still had some aircraft and good radio and line communications with London, making it suitable as a secret location for Biggles to base his operations until he got himself more organised.
Accordingly, Biggles asked Tug Carrington, then driving a taxi, to first dispose of the surveillance and then drive him and his crew to Delmar.
There is no such airfield as Delmar. Biggles asked Tug to take the Watford bypass, suggesting that Delmar is in west Hertfordshire. A possible real life inspiration for the place would be Bovingdon, which is accessible from London via the Watford bypass.