The aircraft was due to conduct a scheduled flight from Aberdeen to Humberside. During taxi it was noted that the flight director annunciator remained it its dimmed condition irrespective of the switch position. The problem was corrected after the switch was recycled three times. The aircraft then made a normal departure. Shortly afterwards however, the tower passed a message to the effect that the landing gear had remained in the down position. The crew noted that although the landing gear selector was in the UP position, the three green indicator lights were illuminated indicating that the landing gear was ‘down-and-locked’. The landing gear selector was recycled, with no effect, and the landing gear control circuit breaker was checked, but it had not tripped. The commander handed control to the First Officer whilst he referred to the Emergency Checklist as the aircraft was held orbiting offshore. The landing gear circuit breaker was pulled and reset, but this had no effect. Whilst the aircraft was still orbiting, with the autopilot engaged, the red Master Warning and the AP (Autopilot) FAIL lights both illuminated, followed by disengagement of the autopilot. About one minute later, the commander’s attitude indicator and altimeter both failed, with an accompanying ‘electrical ozone’ smell. The instruments were restored after their electrical supply was selected to the No 2 inverter, although there had been no ‘fail’ indication for the No 1 inverter. The commander took control of the aircraft and returned to Aberdeen Airport where he was cleared to conduct a ‘fly-past’ 100 feet above the runway in order to have the status of the landing gear checked by observers on the ground. He was then informed that the gear appeared locked-down. Following the subsequent go-around a normal circuit and approach was flown, during which the passengers were briefed on the situation. The touchdown was normal, however during the rollout the gear slowly collapsed. The First Officer shut down the engines and feathered the propellers. The passengers exited the aircraft via the rear doors; no injuries were sustained by either passengers or crew.