If by "the start of it" you mean the far end of it then I agree, that would be about the most exciting thing to look for.. Perhaps it was the Beckhampton Cove (why does the long barrow point straight at it, like all the Stonehenge long barrows point at one or other end of the cursus?) or perhaps it was further out... didn't Stukely say Silbury was precisely equidistant between the end of the Beck Ave and the Sanctuary? There's lots of stones lying about in the target area, including buried ones. I know Pete G has an idea or two (as have I) but he's such a man of mystery now that I don't know if he can be prevailed upon to come here to spill the beans.