Smaller than a mallard. On his own, but seemed to be mated up with female mallard.
No interactions present.
More specifically looks like a cross between a leucistic mallard and a normal mallard. You can tell by the curled tail feathers.
Lat/Lng: 52.144003, -0.715451
OS grid ref: SP879503
Observed in North Bucks Country Park during April & early May