Black ducks on Grand Union Canal, Uxbridge

Can anyone help identify these ducks I saw yesterday. They are larger than mallards, completely black with an emerald sheen. The beak is also plain black or very dark grey, quite long. The photo is not great, what looks like light patches is only reflection from the sun/water. There were 3 together. I can't find anything in my bird books, or on line as yet, to match  . . . Thanks.