Tufted Duck x Pochard hybrid
When I saw this bird, it reminded me about a hybrid duck I'd reported at Linacre back in November 2013. At the time I didn't manage to get a photo, but I did get a bit of poor quality video (see post here). The 2013 bird was also in the company of Tufted Ducks, and also had a large black area on the end of its beak, so could well be the same bird. I also reported a very similar bird at Linacre in April 2010. Again, no photos unfortunately! Interestingly, this same bird (presumably) was reported on the Darley Dale Blog visiting Chatsworth in February 2014 (see here).
Hopefully, if it returns to Linacre I'll get a photo and be able to compare it with this bird.