To me, the bird in question looks like a typical Common Buzzard.
The dark carpal patch, which contrasts with the rest of the underwing coverts, is perhaps the most obvious feature on this bird and is typical of Common Buzzard. The dark lower belly of the Po Toi bird also fits common buzzard, and the pot-bellied structure also, to my eyes, makes it look like that species.
Brahminy Kite should show more uniform wing coverts. They also seem to typically have dark secondaries, contrasting with a paler area on the primaries (even on the paler-winged birds in Geoff's link, the secondaries are mostly dark) - there may be some racial variation in this, but I can't find any pictures showing a pale underwing with contrasting dark carpal area (as on the Po Toi bird).
Obviously this would be a very late date for Common Buzzard - the latest recorded in the Avifauna is 21st April - but does that really make it less likely than Brahminy Kite, with 6 previous records (none since 1990)?
