The pilot lost control of the aircraft during a turn onto the final approach at Fairoaks Airport. The final turn had been commenced from a relatively close downwind leg, requiring a higher angle of bank than usual to complete. In the latter stages of the turn, with flaps at the takeoff setting, the bank angle was increased and there was a sudden and rapid departure from controlled flight that was consistent with a stall. The occupants were able to recover to an approximately level aircraft attitude but were unable to arrest the descent rate which ensued. The aircraft struck the ground and slid for a distance, sustaining extensive damage and causing injuries to both occupants.