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Looking at this again, I think all the ducks are Teal, the males with reddish heads and the females plainer brown, basically. Of course, there are some geese at the back. Teal are small, dumpy-ish ducks and show a white flash along the flanks and a pale area beneath the tail...both of which can be seen on these birds.
My Flickr photos...
Agree with Ian - almost all Teal, with Greylags behind. A couple of drake Mallard visible against the reeds at the back on the left of the right-hand photo.
Flickr pics: http://www.flickr.com/photos/72243477@N00/
Lat/Lng: 52.3068, 0.28
OS grid ref: TL555700
From tower hide at Wicken Fen