Large grey wading bird.

So I was driving to Southampton from Portsmouth and about half way I saw a very large black/dark grey bird in a field which I initially thought was a cormorant but it then stood up and it was about the size of a heron if not larger and had none of the markings a heron has and had the same stretchable gullet as a cormorant whilst being twice the size. I searched and it is possible it was black heron but I’ve never seen one of those before. It was very tall with a very long neck definitely not a cormorant. 

anyone got any ideas?

Cheers