I don't think this is Yellow-legged Gull (mongolicus). On that species, the black on p9 is usually obviously less than that on the outermost primary (p10) (which reaches the primary coverts), whereas on this bird the black on p9 also reaches the primary coverts. The black on p8 is also more extensive on this bird than is typically the case on mongolicus. The white spot near the tip of p10 is smaller than is usually the case on mongolicus.
The fairly well-defined streaking on the nape does not really fit mongolicus at this time of year, and the upperparts seem too dark (though it's a little difficult to judge on lone birds like this).
Given the amount of black in the wingtip I would say this is Heuglin's Gull.
GeoffC
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