I think this is Asian House Martin. Although the rump looks quite large, the tail is rather square (I suspect that the apparent fork is due to moult), the tail is rather short, the dusky underparts with contrasting white throat are typical of Asian, whereas in Northen the underparts are uniform white, the underwing coverts are clearly black which is diagnostic of Asian, in Northern they are grey or brownish and contrast little with the rest of the underwing.
Vaurie (1956) also has lagopodum (which the Lok Ma Chau bird at least fitted) as wintering in southern China. In this context it is of note that the LMC bird was in active moult, so had presumably made a local movement due to the cold weather rather than being on migration.
