The upperparts appear to be too dark for mongolicus (Yellow-legged or, as it's now called, Caspian Gull), while the primary tip pattern, with small and very small white spots on p9 and p10 respectively, is also not correct. Although difficult to see, in both photos the legs appear to be orangey-yellow.
For these reasons, this bird is closest to Heuglin's Gull. Head moult is complete, and so there are no streaks, while primary moult has just finished and the white tips to the primaries are very fresh.
However, there may be some vegae influence in this bird, as the black on both p9 and p8 fails to reach the primary coverts. Usually on heuglini in HK the black on p9, at least, reaches the primary coverts.
GeoffC
