Hi Simon
Where was the image taken?
The bird has features of juvenile Scaup, the broad rounded head and bill shape are closer to Scaup than Tufted Duck (although this feature can change depending on posture), the body also looks a bit bulky for Tuftie although size is difficult to judge from a photo. Juvenile birds tend to have darker eyes than adults, it's also a feature of hybrids which can be very similar to true Scaup. I've put two images below,they are of older birds than the one in your image but if you look at the shape of the birds and also look carefuly at the enlarged view of the bills on the upper right of the image and you will see subtle differences
Cheers, John
BELOW: 1st winter female TUFTED DUCK
BELOW: 1st winter female SCAUP
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