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UPDATE: possibly a juvenile Egyptian goose?
Looks like a Muscovy X something or other
I'd go along with BamBam that it's a Muscovy Duck, possibly a young(ish) one. You can see the beginning of the red wattles at the base of the bill. Their plumage is very variable because they've been domesticated.
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Tony
My Flickr Photostream
We have a number of ducks that look like this one on a local bit of wegtland,as a local farm has a few Muscovy ducks we think maybe these could have mated with local Mallard. WATERFOWL SEEM TO HAVE A HABIT OF CROSS BREEDING,MAYBE JUST TO PLIMOX LOCAL BIRDERS.
Pete
Birding is for everyone no matter how good or bad we are at it,enjoy it while you can