A closer view of the steering arrangement;
It looks smaller than a typical shepherds hut (not that I'm an expert), and the corrugated sides look more like a repair being laid over planking as this interior shots show;
And judging by the wheels it's not designed to move any real distance, just around a yard perhaps.
Here's a photo of the mill, called Barford Old Mill, near to Barford Saint Johns in Oxfordshire;
It's not as cool as Mapledurham (I wonder how many will get the reference!) but still very nice. The undershot wheel and workings are still in situ, although sensibly enclosed and no longer capable of being used;
Paul.