Arthur Chater contacted me to say that the female Scaup which was on Bwlchcryws pond on New Year's Day (and probably one of the 2 there in December) was on nearby Llyn Frongoch on Friday. So Matt, you were right and it was still there today.
On other Trisant lakes there were 6 Whooper Swans including a pair with 2 juvs, one of which is very much paler than the other and I am sure they are the two youngsters which Matt and I have noticed on pond Llywernog next to Nant yr Arian.
(According to Collins, first-winter Whoopers from Iceland do acquire quite an early white appearance.)