I would have expected then to have found a lot more grain if it was a granary at some point. One grain doesn't make a granary. Interesting theory though and I am always intrigued by the interpretations people put on to megalithic structures.
My only other thought is that why would there only have been one at stonehenge? Surely there would be others?
I do love the make-your-own model that you have on your site though.