The aircraft was undergoing its second test flight since construction had been completed. After the test sequence had been completed satisfactorily the pilot positioned the aircraft for landing. When the glider was about 150 feet agl the pilot reported by radio "something broke". The aircraft was observed to enter a steep spiral descent which continued until ground impact. The glider had been built by the owner from a kit of parts which had included the fuselage for one glider type and the wings of a different type. This anomaly was not detected until the wings were being fitted to the fuselage. The aircraft kit manufacturer then advised the builder on ways to overcome the problem. The builder had carried out the modifications but found that the flap drivers did not fit correctly into the flap ends. Plates were then added to the flap drivers to provide more engagement with the flap ends. Following the first test flight the pilot, an approved sailplane engineer, undertook to carry out work on the aircraft to correct various faults discovered during the flight. These faults included problems with the flap actuating mechanism. The alterations were carried out with the wings removed from the aircraft. When the aircraft was assembled prior to the second test flight the pilot apparently failed to notice during his inspection that the flap drivers were not adequately engaged in the flap ends. During the approach to land the left hand flap driver had become disengaged and the flap retracted. The resulting asymetric flap condition led to loss of control of the aircraft.