In Biggles Hunts Big Game, Biggles mentioned Crasher Doyle as a contact of his at Air Intelligence who confirmed for Biggles that there had been no recent reports of unidentified aircraft entering British airspace. This ruled out illegal flights as a means of bringing in the counterfeit money, leaving only the option that the contraband was entering via the regular air services.
He shows up again in Biggles Buys a Watch where he gives Ginger information about unidentified aircraft picked up on radar. Of particular interest was a slow-moving aircraft coming in from the North Sea to the south side of the Moray Firth. This had been going on for the last three consecutive months, always on the night of the full moon.