Well hidden female Mallard, possibly on nest
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Unfortunately it's not really possible to be certain because not much of the bird is visible in the photo!
I would say this is mallard nest - the habitat looks fairly common for a good nest spot and is just out of reach for people/predators. The only way to be sure though is to inspect the nest when the adult mallard is away from the area but I would suggest this be done by someone from the RSPB or from the nature reserve in case someone thinks you're egg stealing!!
Saw what I think was the same mallard at the same spot, with 6 little ducklings.
Lat/Lng: 53.910633133726, -1.6613817214966
OS grid ref: SE223461