Yes, it's been reported as a Common Eider x Mallard hybrid ( not sure how they knew). See HERE. It seems to have been hanging around in the North-East of England. Is that where you saw it?
These Mallards will mate with anyone given half a chance.
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Tony
My Flickr Photostream
Thanks TeeJay. Yes, spotted it in Seahouses harbour alongside eiders. By the look of the bill and general shape I thought there must be mallard in there....but cannot see the eider link at all! Cheers, Steve