© Brian Stone
This Gull looks to be a full adult. It gave superb views. Primary moult distinctly more advanced than other large gulls present with P10 well grown but still well short of P9 which is also still growing. P10 shows extensive black before white mirror and no tongue is visible. There is a black band between the mirror and white tip. P9 has no white mirror and P5 has strong black subterminal mark. Closed wing shows 3 well spaced white primary tips and the rear end looks attenuated even at this stage of moult (P9 and P10 will grow to produce a longer wing tip eventually). Dark secondaries and inner primaries are also visible here. Also has a good, sturdy, heavy-tipped bill.
What cannot be seen from these images is the bright orange/yellow legs and the mantle shade. Direct comparison was possible with a number of adult argenteus and graellsii and it was mid-way between the two, perhaps a shade darker than Common Gull (although none nearby).
Helpfully, this bird is at pretty much the same stage of moult as the first illustration on p39 of British Birds Vol 90, Nos 1&2 (Jan-Feb 1997).
Some have commented that the head streaking looks quite extensive. It is within the range for michahellis but could also match atlantis.