All the Gulls! What am I still missing?

I was looking through my ancient bird diaries today and realised that I've managed to see almost all the gulls you could expect in the UK: most of these in Norfolk in the fifty-something years I've been birding here. Since I only started carrying a camera a few years ago, there are one or two species that are represented by images from the above-mentioned log books!

The most obvious omission is Auduoin's Gull: I've seen one in France (in Boulogne Harbour!) but never in the UK. I've only managed four Ring-billed Gulls in the UK: one of them I actually found at Cley! I ran to the old Coastguards to inform Mark Golley, who not only confirmed it, but also submitted it to the Bird Report (as did I!) When the report was published it said:

Ring-billed Gull The 1991 second-winter bird at UEA Norwich remained until April 7th. A first-winter at King’s Lynn docks Feb 16th (AB CD), a second-summer at Cley May 1st (DB MAG et al), a first-summer later moulting into second-winter at Breydon/Yarmouth July 3rd into 1993 (DJH et al) and a first-winter at Walcott Dec 21st (MF). 

The odd one out among the images is the 'dark Herring Gull': a lot of very good birders considered this as a strong candidate for American Herring Gull...

Iceland Gull, Cley

Glaucous Gull, Cley

Lesser Black-back, Blofield Heath

Greater Black-back, Cley

Ross's Gull, Topsham

Ivory Gull, Aldeburgh

Herring Gull, Bempton

Possible American HG, Ludham

Yellow-legged Herring Gull, Lingwood

Caspian Gull, Minsmere

Common Gull, Buckenham

Ring-billed Gull, Hayle Estuary

Franklin's Gull, Titchwell

Laughing Gull, Walcott

Bonaparte's Gull, Morpeth

Little Gull, Kelling Water Meadows

Sabine's Gull, Scillonian Pelagic

Kittiwake, Sheringham

Black-headed Gull, Hemblington

Mediterranean Gull, Gt Yarmouth

Slender-billed Gulls, Cley