The features that immediately stood out were the small white 'sails', a sure sign of Northern genes. These appeared to be darker-tipped on their undersides (presumably indicative of a sub-adult bird) and were more prominently raised when the bird was chasing the nearby females around. The bill was orangey-green (more orange towards the base), not quite the 'classic' borealis bright orange-yellow (you might recall an Eider with a bright orange bill that appeared at Rhos around this time of year for a couple of years running, but it never showed any sails). I also noticed that the green patches on the nape were paler than on the other drake Common Eiders (not sure if this is a recorded feature of Borealis or not?). The lateral crown stripe was brown (not white) and I assume that this also points to the bird being sub-adult. It was displaying with the other Common Eiders (although they seemed to take quite a bit of offence to it!) and it appeared to be slightly larger/bulkier in comparison to the other drakes.
I hope the photos are of interest - with a resident group of Eiders in our area it's well worth keeping an eye open for these types turning up amongst them in the future.
Cheers and good birding!