Roy, you really need to get the Stripple stones henge very nearby, as i'm sure it will give you a better understanding of King arthur's hall, apart from the shapes they are very similar, and it may have been a case of "they've got a circle, we'll have a rectangle", I imagine the builders of King Arthur's Hall came from France, because in Brittany it wouldn't be out of place at all, the circle henges are all connected and a route may have gone all the way to stonehenge [and way past] with it's own rectangle [The station stones] and it's other Brittany symbolism.