The sun first shone onto the floor, before illiminating the blocking stone directly at its base and rear chamber by scattered light. The sun finally disappeared at 15:31. The light inside was the brightest the author has ever seen, partly due to reflections off water in the passage and partly due to the elevation of the sun.
Pictures were taken on a Kodak DC50 digital camera, on loan from David Griffith of Orknet. Many thanks for this very helpful gesture.