This adult bird shows a pretty distinctive wing formula which is typical of Armenian Gull (Malling Olsen and Larsson pg 26 and pg 300-307). Only one mirror on p10 and black extending all the way to the primary greater coverts on p10-p9 (p8) ((p7?)) on atleast the outer webs (outers and inners on p10-p9 (p8?). This produces a 'step' in the black on the wing- from p8/p7 onwards. The black on the remaining primaries also extends to p4 and sometimes p3. To me that is quite a distinctive and arresting wing pattern. Combined with the size (smaller than Yellow-legged), the dark eye in adult plumage, the four coloured bill (pale tip, red gonys, black ring and yellow basal section) and the relatively dark upperparts- this is pretty distinctive gull (most similar to 'Steppe Gull'). There are suspected recent (last few days/weeks) claims of this 'species' in both France and Italy.
Other Armenian Gull photos and info:
Needless to say, my experience of Armenian Gull is restricted to a few weeks observation, therefore anything I have to say on the matter should be considered with appropriate caution. Any comments most welcome.