I was dropping a mate off at Filton once, he had never flown in a light a/c before and he had sank a couple of pints before we took off. As the journey progressed he got more & more desperate. As soon as we were on the runway at Filton he needed to get out. I told him we would have to taxi clear of the runway. I tried to make it to the parking area but as we passed the tower he said he really couldn't hold it any more. I don't know what atc must have thought as we shut down right in front of the tower and a large skinhead jumped out of the plane facing straight towards them and relieved himself all over the taxiway!! (I always carry a red bottley thing now)