I've only just transferred this photo from the camera to the computer. It was taken a month ago on 8th December when I noticed what I think was a weather-front, a big bank of cloud, approaching and then going over Ampthill Park. It was pretty impressive. You can just see Katherine's Cross on the top of the Greensand Ridge. The photo was taken, late afternoon, just north of The Rezzy.
Update: Angela, our Meteorology Recorder, thinks that this formation is Stratoculumus Undulatus, which is described as patchy sheets or layers with parallel undulations.