Did a couple sorties at one of my local flying spots Collieston (North East Scotland) on my second flight I flew out to a ship anchored offshore. Flew back to my take off spot with no issues about 40 seconds later a light aircraft flew past about two to three hundred meters offshore at about 20 meters off the surface of the water. Gave me a fright as I believed there should have been no chance of me flying in airspace that could be occupied by light aircraft. I have it on video and will post it later, I dread to think what would have happened if he or she hit my drone head on as he or she was flying at least 80 knots. I flew in accordance with Uk laws average height was 50 meters and myself or spotter had her in sight at all times. Who was at fault If there had been a collision? Speed & height they were flying at there is no way I could have taken avoidance action. We were outside controlled airspace I'm not sure what that means in terms of flying a light aircraft.