I saw a newspaper report about this bird! I often go birding in the area and both Common Buzzards and Marsh Harriers are around in good numbers. Dark brown underside could make it a dark Common Buzzard but would also fit a female-type Marsh Harrier. Another possibility is an escaped falconer's bird, with Harris Hawk the most likely (very popular esp. among amateur falconers). I have seen a lost Harris Hawk at Dungeness, and again a dark brown underside would fit that species: http://betterphotos4you.com/wp-content/uploads/2011/08/Harris-Hawk-in-flight-2.jpg
Talking of Harris Hawk, there was a similar report on the Community a couple of weeks ago which was probably a Harris Hawk in Amersham see HERE