What was hoped to be a fairly straightforward trip through Priddy Green Sink and out of Swildon's turned out to be a bit of an adventure. All was OK until the main 20m big pitch. I went down first. Paul then followed but somehow lost control of his Petzl Stop which didn't stop him and he plummeted about 10m before landing on a ledge with his knee. A very loud scream later and his petzl stop and stopped him on the rope. Upon seeing him fall I grabbed the bottom of the rope, he was now in a lot of pain about 10m from the floor. At this point I wasn't very sympathetic and shouted at him to abseil to the floor (there wasn't much I could do while he was hanging 10m from the floor). It was soon clear that he couldn't stand on his leg. I examined his knee and leg under his oversuit and fortunately there was no blood or protruding bones. Helen then came down to join us. The decision was then made to move Paul down the next two pitches so as a rescue attempt could be made from the Swildon's end as apposed to more people having to come through the unpleasant Priddy Green Sink. I then left Helen and Paul one pitch away from the Swildon's Streamway, which I had rigged so as people could SRT back up, and headed back to the surface via blue pencil passage and the short dry way. Once on the surface I went to Brian Prewer's house to start a formal cave rescue via MRO. I met some other cavers at the belfry, explained the situation and then headed back to Paul and Helen with Joel and Phil carrying a first-aid kit and underground radio. Successful communications were made with the surface and we got Paul to sump 3. Here we were met by some other divers who took him out through the sumps. Helen and I then headed out of the cave via blue pencil passage and the short round trip whilst others took care of Paul. The ambulance met Paul on the surface and took him to Bath A&E where the X-Ray showed he had broken his kneecap into many pieces.