It is with, but slightly apart from, the other Tufties and the white ring on the bill really shows up. The other id features are all there for comparison with the female TD's, tapering bill, longer tail, paler warmer brown tones and a distinctive eye ring with pale line leading back towards the straight back of the head. Its crown is described as peaked not crested and it certainly looks different.
It flapped its wings once to reveal the grey wing bar so possible to pick one up even in flight.
Numbers of them have become trapped on this side of the Atlantic and migrate up and down the coast of Europe, a flock of 15 being seen in Ireland so maybe some have bred. Of course being ducks they do interbreed with Tufties but this lady looks quite pure and showed no interest in the two males who were following her around!
Memo to John: this sighting by Arfon reminded me (as I was hiding indoors enjoying a late breakfast) that the best birding is just to do it more often!