Seabird Unknown

After my swim this morning I popped down to the harbour with my telephoto lens looking for possible shots. The harbour was unusually quiet with only a smattering of fishing boats and the lifeboat in dock and  surprisingly few gulls around. I went up onto the harbour walls to see if there was more sea birds around on the groups of rocks that surround the harbour entrance but it was equally quiet. However, I did spot four sea birds that I did not recognise on one of the rocks and took a few shots of therm. They were low down on the sides of the rock just a couple of feet up from the sea. I spent quite a bit of time this afternoon trying to identify them using my Collins Bird Guide and the internet but failed miserably. I wonder whether any of you can help out here. They are slightly bigger, or at least fatter, than the average gull and have large webbed feet. Their tiny eyes are hidden on the edge of their black cap and they have a yellow beak. Most of the birds have dark wings and whitish breasts but their plumage is the opposite way round - a dark black breast and pure white wings.