In reply to Climbing Pieman:
Nice Eider piccy - they are beautiful and very obligingly visible from a great distance with that plumage.
General rule with similar looking species - it's the most common one unless you've got some specific "evidence" to the contrary. Not always correct of course.
Also, sometimes (too often actually) you'll just never know which bird it was; I've got several at the moment where I basically still need a definitive view to be sure; e.g. the two partridge that flew past me the other day; grey or red-legged - I think grey but didn't have time or a good enough view to be sure, and that made me wonder about a previous sighting that thinking back, wasn't definitive between the two.
And then I think - if I couldn't see/hear it well enough to be able to definitively identify, then was it a worthwhile sighting? And my answer is no, I want a better sighting than that.