The "Spur" was a advanced long range fighter used by Biggles in Sergeant Bigglesworth C.I.D.. According to novel, at the time of the story, there was only one Spur prototype which was still undergoing tests. The aircraft was a twin engined two seat high performance fighter which was "only prevented from making history by the termination of the war." There are few other technical details of the Spur, except that in the chapter "Gloves Off" when the Spur comes under attack by the Renkell Wolf, there is the mention that "Ginger had started to turn, to man the gun turret". This suggests that Johns was imagining an aircraft along the lines of a Beaufighter or Bristol Brigand.
The Spur is, of course, a fictional aircraft. Johns was pushed for choice, after all. He needed an advanced, late war design, something modern enough to take on the Renkell Wolf. It had to at least match the German fighter in range, agility and firepower. At the same time, it had to be a two seater, or else there would not have been enough space for his whole team. There was no real aircraft fitting this requirement.
The closest parallel for the Spur would be the de Havilland Hornet single seat long-range fighter. It is possible that the Hornet inspired his choice of the Spur. The Hornet, like the Spur, was a late war design which entered squadron service just too late to see any combat. Like its predecessor the Mosquito, the Hornet was made largely of wood. It had very long range, and in its time, was the fastest twin engine piston aircraft in R.A.F. service. Although it would be rapidly overtaken by postwar jet fighters, the Hornet rendered good service with the Far East Air Force where it served extensively as a strike fighter during the Malayan Emergency. Its long range gave it excellent loiter time, allowing it to provide fire support to Commonwealth troops fighting communist insurgents in the Malayan jungles.
A naval version of the Hornet, the Sea Hornet, also provided many years of good service as a carrier borne fighter. A two-seat night fighter version did in fact exist, but the operator sat in a separate cupola some way behind the pilot, in the rear fuselage. In the "Spur", there is a suggestion that the crew sat in tandem, as Ginger could see what Biggles was doing but this would seem to contradict the idea of him turning "to man the gun turret".
When Francis Bergèse had to visualise a "Spur" for his graphic novel adaptation Le cygne jaune, it is likely that he based his "Spur" on a Hornet, with an elongated bubble canopy to seat two crewmen in tandem. Certainly the tail section of his "Spur" looks very similar to that of a real Hornet.
The Spur as imagined by Francis Bergèse in Le cygne jaune. The tailfin is squared off on top but compare the fillet on the tailplane with the Hornet below. Note also the very obvious Merlin engine nacelles as mounted on a Hornet. As noted in the article on Le cygne jaune (Miklo), the registration G-AGZZ belongs in real life to a Tiger Moth. There is also no RAF serial XFDI or XFD1.
A de Havilland Hornet for comparison.