One calm September evening in 1966 (a year after the film premiered), I was doing dual circuits in a Piper Colt at White Waltham. During one downwind leg, we noticed a strange-looking biplane tracking slowly north to south at about 500'.
My instructor took over and we went to have a look. It was the Bristol Boxkite. My instructor tried to formate but could not. The stall speed of a Colt is about 50 mph and the Boxkite's maximum speed was 45 mph!