The whole story, as usual, from Colin Cummings' excellent book Category Five:
7 Oct 1958 WP868 Chipmunk T10 CFS Near Bibury Gloucestershire
The aircraft had been airborne for about 7 minutes on a sortie to include practice circuits, landings, turns, stalls, spins and aerobatics. It was seen to drop its starboard wing and spiral into the ground from a height of about 600 feet. The reason for the accident could not be determined, although it is possible that one of the crew undid his harness straps to reach forward to an inaccessible switch and in doing so pushed the control column forward.
Squadron Leader William Edward CALDECOTT 40
Squadron Leader John Stewart LAMPLOUGH 43