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Doesn't seem very Whitethroat-like. Any other pics available? Maybe just the light coming through the leaves, but it looks rather grey-green tinged; so the back doesn't look very chestnut for instance. Tail also looks odd... As I say though, may just be the poor light and not-sharp image.
Flickr pics: http://www.flickr.com/photos/72243477@N00/
Would agree, does not look very Whitethroat like to me either.
Long undertail coverts, structure and colouration suggest Reed Warbler
Lat/Lng: 51.352, 0.827
OS grid ref: TQ969652