As we continue to wait for the analysis of the Mumbles erratic, which was claimed to be the "smoking gun" of the Stonehenge Bluestone Glacial Transport idea, it is worth reconsidering the pantheon of coastal erratics in the southwest of Britain. Conventional glacial theory says they shouldn't be there, but they are, so the theory needs refining. That they were entombed or carried on icebergs seems to be the most probable answer.
Quaternary of South-West England edited by S. Campbell, C.O. Hunt, James D. Scourse, D.H. Keen, N. Stephens
The good Dr John, however, seeks inadvertently to confuse: "Whatever its erratic history may be, the boulder demonstrates that the Irish Sea Glacier impinged upon the Gower coast, carrying erratics from the west and displacing local Welsh ice on at least one occasion."
The source of the erratic is important, if it was entrained in a glacier and dropped by the glacier then the source and destination must be connected by a plausible path. But if it sailed free in or on an iceberg then the source could be unconnected, and its position doesn't demonstrate the extent of the glacier at all.