it's rideable on the unmade byway from the bottom of willoughby road. I wouldn't do it on 25s, but I've done it on 30mm slicks before. however, what you do after the stones, I'm not sure, the byway continues to the A303, and beyond, but never really approaching the road the route is on.
return to willoughby road, and on to woodhenge back to the route? Or bash along the A303, because it will be 2am and empty?