Since posting my query, I've discovered an aerial photo of the Rings taken in the early 1920s showing the buildings looking very neat and military - it's in Wessex from the Air, by O G S Crawford. A service road looks as it's touching the outer ring. I now think the buildings were to do with the vast experimental area created in the Great War at Porton Down, which spread as far as Winterbourne Gunner; the modern military base is less than half-a-mile to the north. Frustratingly, the Cunningtons, I think, were excavating the site c1925 and described their work in the Wiltshire Archaeological Magazine, but, I'm told, did not mention the buildings. The 1901 OS maps show no buildings at all.

Moonraker