Steve,
A 2-flagged godwit would typically have both flags on the tibia and this is the procedure in HK. It is therefore unlikely that the bird you saw had a Hong Kong (white over yellow) combination. A single white flag should indicate that the bird is from New Zealand. You mention that numbers were not visible - does this mean that numbers were present but unreadable or that the flag was plain?
The black over white combination on the Great Knot indicates a bird flagged at Chongming Dao (near Shanghai).
Check this website (
http://www.eaaflyway.net/documen ... ebird_flag_2011.pdf) for details of the flagging protocol along the flyway, and be sure to report your observations ( the best e-mail address to use is probably: mintons @ ozemail.com.au).