This is the windmill that you can see in Spencer's first photo from Ivinghoe Beacon. It's a 'post' mill, so called because the whole tall top can be rotated around a central post, thus taking advantage of any wind from any direction. It was built in 1627, restored by volunteers, and is in the care of the National Trust.http://www.nationaltrust.org.uk/main/w-vh/w-visits/w-findaplace/w-pitstonewindmill/
Windmills and watermills are very ancient edifices; they served a whole village and would have been a prominent feature pre- Enclosure. I don't really know what happened to milling arrangements after the Agricultural Revolution, but it's not something that you would expect to find in every farm.
The picture is from Flicr:
http://www.flickr.com/photos/skinnyde/21475105/
And very pretty it is, too!
