I have just realised that there is a late 60s, 1968 I think, children’s television drama called The Owl Service, based on a novel written by Alan Garner, which deals with this very story in a different kind of way. It doesn’t actually tell that story, instead it tells the story of some young people who are having issues who begin to re-enact the story of Llew, Bloduwedd and Gronw. They start to act like the characters and become very opposed to one another. Part of this activity revolves around a stone which has a hole in it which is supposed to have been pierced by the spear which Llew threw at Gronw. Indeed one of the characters briefly thinks that they see Llew standing posed with his spear upon a hilltop. Strange owl like sounds and mysterious events surround all of them.