This is a first-winter
Larus (cachinnans) mongolicus (Yellow-legged Gull), and is probably the same as the second individual at the following link:
http://www.hkbws.org.hk/BBS/viewthread.php?tid=9793&extra=page%3D1
The tail band is too narrow for Slaty-backed, on which it usually covers most of the tail. The overall colour of the bird is rather pale, which is not correct so early in the first winter.
While the bill is, as you say, quite large, so is that of
mongolicus.
Good photos!
Geoff
