Having convinced myself that yesterday's bird was probably just a Sand Martin, based on the apparently warm colouration, I received the following reply to my request for help from Paul Leader in Hong Kong (reproduced here with permission):
"These are Pale Martins. Structurally Sand Martin can be easily eliminated by the very short shallowly forked tail. On Pale Martin the tail when slightly spread is square, on Sand Martin there is still an obvious fork. In one photo at least I can see five tail feathers on either side of the central pair, so there is no tail moult.
In the field the breast band on fokhiensis (the 'Pale' Martin with which I have most experience)often appears darker than is illustrated in field guides or I would expect from skins. The lack of a marked breast also eliminates Sand Martin, note also how narrow the breast band in on one of your birds.
The structure at rest is diagnostic as you state [the wings extend beyond the tip of the tail]. But do not get too tied up in overall colouration, note in your birds the marked difference in colour in the freshly moulted primaries and the old ones. I have found light conditions also have a strong influence on perceived plumage
tones in Pale Martins.
I would have assumed that these were fokhiensis, however these have obviously dark underwing coverts which fokhiensis lacks and in fact fits better ijimae Sand Martin. I can think of two explanations for this:
1) fokhiensis has dark underwing coverts when they are freshly moulted and
2) these are not fokhiensis but another taxon from the diluta complex e.g.
2) seems more likely."
Paul has also identified this martin I saw in December as Pale Martin, with the following comment:
"I think very little attention has been paid the Pale/Sand Martin issue in SE Asia. In HK for example, it appears as though Pale Martin is much more common, with the exception perhaps, of late spring."
Here are some more pictures of yesterday's bird: