Laysan Duck x Mallard Anas laysanensis x Anas platyrhynchos
Whilst it is not hard to see Laysan Duck influence in this bird, its appearance is so far from a normal Laysan Duck that I was convinced it must be a hybrid. It transpired that it's actually a "ginger" variant of Laysan Duck that's sometimes kept in wildfowl collections. Exactly how these are produced I am not sure, but it sounds like they probably contain some Mallard blood.
Thanks to Joern for clearing up the mystery on this one, and for finding this photo of a pair of similar birds in a Scottish collection.