Strange Duck

Zoe and I have just got back from our Sunday morning walk around the Reading University lakes. There we saw a rather unusual looking duck.  Zoe first noticed it sitting on the lake by itself in the far distance.  When Zoe started throwing bread it sedately swam up close to us so we were able to get a good look.  Its head and neck were white and the rest of its body was black with a greenish sheen.  The boundary between the white and black was not sharp, nor was it smoothly graduated, instead it was spotty.  This lead me to think the duck was a partial albino mallard or else a hybrid of some sort.  However, it was not associating with any of the other mallards on the lake. Also, the upper base of its beak was bright red, its legs were yellow, and its back also seemed rather too broad for a mallard.  Maybe it was a shelduck hybrid?