Dropped in on the reported "male scaup duck" at Rogerstone this afternoon. As anticipated, it wasn't a Scaup but it was more interesting than it might have been, cynical ol' me was expecting a Tufty. The bird appears to be an escape, it allows very close approach and readily swims to within a few feet for fodder, I don't think any rings were present though. It looks pretty good for Tufted x Scaup but, given a captive origin and the variation in mixed aythya offspring, back-crosses, etc., I suppose ruling out input from Pochard, Lesser Scaup or even something as exotic as New Zealand Scaup, etc., might not be possible.
Just in case you want a little more detail the bird is larger than a Tufted and quite broad in the beam. The bill is long and broad with an extensive black tip, a sub-terminal white 'crescent' and an inverted black, ill-defined, 'U' on the culmen on an otherwise blue bill (no significant darkening at the base). The iris is medium yellow. The head has a quite steep forehead, short blunt crest and extensive green and purple sheen (green largely restricted to the ear-coverts, purple largely restricted to forehead, crown and nape); overall the profile is much more Scaup than Pochard-like. The mantle is finely vermiculated blackish, appearing medium grey at a distance; tertials black with greenish gloss; flanks very finely vermiculated pale grey, appearing off-white at any distance. The breast, neck, uppertail, tail and undertail all appear black. It didn't wing stretch during my short visit; a pic of an open wing would be useful should anyone feel inspired to get one.