For centuries, stories have circulated that the Knights Templar possessed the Holy Grail and other sacred artefacts. They were said to have unearthed the cup that held the blood of Christ in Jerusalem. The origin of this tale is very likely in the tales about King Arthur. So why are so many people convinced that the Templars owned the Grail? Talking Templars presenters Dr Steve Tibble and TV historian Tony McMahon investigate.