Black Duck?

Late afternoon at Marloes Mere I discovered an interesting looking female duck, looking pretty good for a (North American) Black Duck. It appeared paired to a drake Mallard, and they were feeding very actively at the far W end of the main water body, more or less in front of the new Britton Hide beyond the Youth Hostel. I was joined by Steve & Anna, but we weren't able to clinch the deciding factor - a purplish/blueish speculum with no leading white edge. On other plumage features though, it looks very good, including an all dark tail - but I will be happy to be proved wrong. Richard D is going to have a look in the morning.

Also here at least 8 Sand Martins, a male Wheatear, and a feral Snow Goose in with the