The left seat pilot was undergoing his second session of endorsement training in the aircraft. After a cross country exercise under simulated instrument flight conditions, the aircraft returned for a landing on runway 11 Left. The pilots reported that the undercarriage was selected down on the downwind leg, and confirmed by three green lights and the left engine nacelle mirror. The approach and landing were reported to be normal, with initial touchdown on the main wheels. Soon after the nosewheel contacted the ground, the nosegear collapsed. Examination of the nose gear downlock mechanism, indicated that it had been subjected to considerable forces which could only be attributed to a heavy landing or landings. These forces had dislodged the downlock roller and stop bolt away from the downlock hook. Subsequent forces then allowed the nosegear to retract and hence collapse.