These are definitely Common Tailorbird. As suggested by thinfor, Rufous-capped Babbler would be bulkier (less slender) with a shorter bill, shorter legs, shorter tail and yellower underparts. The behaviour also fits better for tailorbirds - the babbler would usually be in dense vegetation and fairly mobile.
Rufous-capped Babbler also calls frequently - a distinct call of five notes, all on the same pitch, lasting no more than a couple of seconds. Tailorbird, on the other hand, could give a continuous song for a period of 20-30 seconds as described.
Rufous-capped Babbler is now numerous and widespread in shrubland/forest of the central and eastern New Territories but rarely (if ever) recorded elsewhere.
