A fellow brochaholic, an actual archaeologist, has noted the narrow neck to the promontory that this stands on as strongly resembling other broch sites he has visited, saying that if it isn't a broch it has to be something else significant.
Perhaps if it weren't so easy to get onto the 'Viewpoint' site (just over a turnstile from the new Viewpoint and often a little damp to reach) wouldn't appeal but it is a significant feature in this landscape, as much I feel as Maes Howe (which is only just down at the junction of this Stoneyhall Road and the Kirkwall-Stromness road). From the depression at the top you look out lochwards and see how heavily humanised even this view across the holms is. In the loch edges on either side are many stones, and just off the area which leads into the main body of Harray Loch can still be seen an obvious rectangular structure.
An archaeologist friend of mine who had visited this site said perhaps it did not appear in the records because rather than a broch (which had been his impression too) it was actually the site of St. Mary's church that had somehow been displaced on the map. However on closer inspection during my second visit I found a very small piece of exposed structure, only two or three thin slabs but feeling more prehistoric than ecclesiastical. Being not all that observant I missed a relatively big chunk of stone in the remains of a trench on the side until my last visit.
Going back onto CANMAP today I find on the next promontory along as well as the remains of St. Mary's there is also shown the Grimston (alias Biggings) Broch which I'm sure wasn't shown last time I looked, so now I shall add this to Orkney as a positive.