At Brandon Marsh nature reserve. Pretty sure it's a Teal, but as I've not seen one before, I could be wrong.
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...and isn't it a small duck. A good first spot and you were absolutely right. Should make identifying one the next time easier.
Feel free to look at my ongoing (hopefully improving) collection of pictures on Flickr:
Lat/Lng: 52.375, -1.4312
OS grid ref: SP388753