does "very accurate" signify a heightened likelihood? ;)

I don't think so , a couple of degrees out would almost make it more likely

In musing about this elsewhere I've come up with a further possible dimension - literally. See this - Silbury from the Sanctuary, with Silbaby hidden by the trees - http://www.themodernantiquarian.com/post/19965

have you checked the grid ref derived from GPSU to see if it matches Silbaby