I don't think this is a phyllosc at all. the jizz is all wrong.
It gives the impression of a bush warbler of some sort. Initial impression is that it's similar to a stubtail, but obviously with a longer tail.
I think that this might be pale-footed bush warbler Cettia pallidipes. It's very similar to the bird in this link:
http://www.orientalbirdimages.org/search.php?action=searchresult&Bird_ID=1841
Note especially the long, pale supercilium, dark eyestripe, dark back and flanks and pale legs. The second photo also suggests that this bird only has 10 tail feathers, which would be right for a Cettia.
According to the avifauna there are only 7 records in HK, all of birds trapped, and none since 1998. I think there have been claims since then but I don't know if any are proven.
Where abouts did you find this bird? I'm sure others would be interested to see it!
And congratulations on such a good set of photos of what is obviously a rather skulking bird - the last is one of the best bush warbler photos I've ever seen!
