Dark mantled lesser black backed gull

Birding on the river Clwyd this morning I came across a dark mantled lesser black-back type gull which was noticeably different as it stood next to our 'normal' graellsii lessers. Apart from the dark mantle the bird showed obvious longer legs and a longer bill than the graellsii. The eye appeared pale. After a bit of research I would put down as an intermedius rather than any cachinnans type but without my camera (packed away after just returning from week in Croatia). I guess it'll remain unsolved - unless it's back tomorrow and get chance to photograph it. Three whimbrel and a lesser whitethroat also around.