I'm going to throw something else into the pot, though I'm on the side of the bluestones already being at Stonehenge..
If you look at Bedd Arthur, its horseshoe shape is very reminscent of the horseshoe shape at Stonehenge, which might be a coincidence but Bedd Arthur is opposite to Carn Meini (though not facing it) and is undated...Prehistoric Preseli - Figgis

http://www.themodernantiquarian.com/post/45 the only photo that shows the actual shape!
Atkinson in his book Stonehenge, says that a (again quoting) bluestone boulder was found in the second section of the waterway route. Someone else says, that the "Flemings Way" was originally a prehistoric trackway, so could have been the overland route to E.Cleddau, but no one is going to drag/float heavy stones for 180 miles....