My Castleden's book arrived this morning.. he says south stone is only 0,75 m wide and only 0.22m the northern stone being shorter... it could be a single longstone (as proposed by Engleheart in 1933), which may have been similar to the altar stone, but it could well have been an Avenue stone.. The Engleheart theory was written in article form in the Wiltshire Gazette. Only putting the avenue stone theory because theres a heavy argument in favour of "altar stone" whereas it could have come from somewhere else (I do like an argument to be balanced ;)