Please help me identify this bird

Seems to be nesting on the side of our farm pond but I've also seen it swimming on the pond and feeding by sticking it's head beneath the water (but not diving).  It looks completely black but I thought it had a white throat that I'm sure it inflates from time to time (my wife thinks so, too, so it's not just me).  About the size of a mallard - maybe slightly bigger (that's a blackbird just landing on the fence post in the second photo, to give you some idea of scale).  We live in SW Scotland about 5 miles from the sea on a river estuary.  Have seen cormorants flying up and down but didn't think they had white throats.  

Dave.