Hi Sammy,
I think it should be a Pale Martin.
Please take a look of the comment by Paul LEADER one year ago.
http://www.hkbws.org.hk/cgi-bin/yabb/YaBB.pl?board=BirdingTips;action=display;num=1144158702
http://www.hkbws.org.hk/cgi-bin/yabb/YaBB.pl?board=Swifts_Swallows;action=display;num=1145366679
Regards,
Forrest