Yes, sorry for the hurried reply.
The bird is either a (1st-cycle) Mongolian gull or a pale Vega gull resembling Mongolian.
The barred greater coverts, showing even a slightly stronger pattern than the median coverts, are a big clue;
in Slaty-backed, the median coverts contrast darker against the paler, more washed out greater coverts.
Then there is also the pale basal colour of the scapulars, with a few strong, transverse dark bars, and the rather straight bill shape (not as bulbous as in typical Slaty).
The white head shows just a dark eye line, not a dark mask, and contrasts with crisp, dark streaking on the lower hindneck.
The overall shape of this bird also looks a bit off for Slaty-backed to me, with the secondaries covered by the greater coverts and with such a flat back; Slaty-backed usually has a bulkier, more oval-shaped body and its secondaries are usually visible on the folded wing.
