Back to the pond

I've seen this duck each time I've visited the pond during the past month. It is the only one with this appearance, all the rest are Mallards or Canada Geese, plus a few gulls which appear when people are donating their stale bread to a good cause!!

It sits on the island and drinks from the pond. I have not seen it in the water whilst I've been there.

Not sure what it is, but I'm sure someone can help me out. I have looked in my bird books but without success!!

Many thanks in anticipation of an answer :0)