Sounds about right. The local archeology guys tend towrds it being natural/etc but with the question over one of the stones. It may not be local - it hasn't been checked out. If it is local it looks pretty cut and dried as the only 'evidence' that it isn't is that stone. Vice versa if it is not a local stone. Although I live close I havn't visited the site as my efforts have been elsewhere. Howver I hope to change that soon and get some high definition digital photo's of the stone in question.
What concerns me about all this is that the information that I found is readily available. Perhaps other members should take a cautionary pause. How many sites are there listed here that are accepted by members but no one has actually asked the question of authenticity? Perhaps new entries ought to have an authenticity note added?

In the old postcard picture those small blocks lying underneath might perhaps have been 'field clearance' stones. The larger ones seem genuine enough. Oddly I have a hand axe, of chipped Greenstone, that is very similar in shape to the capstone. As above, so below ?