Or more accurately, ex-carp. it's dead as the proverbial doornail.
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It's a bit too large - bearing in mind it's behind the swan, and not all of it is visible. There areen't that many freshwater fish that aren't recognisable in some way or another - we can rule out pike, for example.
This fish is rather rounded. Too big for a chub, wrong habitat for barbel, wrong colour for tench (unless they change colour when they rot).
Though it's not the best way to identify things, by a process of illimination I think carp's the most likely.
Trout will grow pretty big in Grafham! Both Rainbow and Brown will be well into double figures. You may be right with carp, but it could just as easily be a trout, to be honest, especially as dead fish commonly 'blow up' and assume that rounded appearance. A large Bream would be a possibility, too.
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Lat/Lng: 52.29546, -0.34134
OS grid ref: TL132676