The Freckled Duck, a distant relative of geese/swans, is a rare native of southern Australia. This one is in Barnes, SW London.
The drake (above) has pink at the base of his bill and goes darker with a particularly dark head in breeding plumage.
The duck (female, below right) is a lighter brown, similar freckled pattern, without the pink on the bill.
Freckled Ducks have a characteristic "corner" on the top of their head rather than a rounded shape.