The pilot was preparing to depart from a strip which was only marginally longer than the minimum length required. He reported that when take-off power was applied, an overboost warning light illuminated. Although a considerable power reduction was required on the right engine before the light extinguished, the pilot continued with the take-off attempt. He advanced the right throttle to match the position of the left lever, but as the aircraft became airborne it struck the boundary fence and the left mainwheel was dislodged. The pilot elected to return for a landing at the strip. During the landing roll, the left gear leg collapsed and the aircraft ran off the side of the strip. No fault was subsequently found with the engines and associated systems of the aircraft. The overboost warning light system fitted to the aircraft type is characterised by a significant lag during power reduction. The normal method of re-setting power under these circumstances is by reference to the manifold pressure gauges. Although the strip was marginal for the intended operation, the engines were not advanced to a high power setting before the brakes were released.