Maybe have a google for winter plumaged Black Guillemot and see if that's a match?

Yesterday at Moelfre I watched a small black and white diving bird with bright red legs. All I can think is that it was a Black Guillemot, but the markings were not what I'd expect - more black than a juvenile, but more white than on an adult... Is there another diver I'm not aware of with that colouring and bright red legs??