The image of a juvenile Lesser Frigatebird in Harrison's seabirds also looks a good match - Mike T's comments about the extent of black on the belly, and the shape of the spurs seems to point in that direction.
Geoff, since I didn't see it makes no difference whatever which one it turns out to be!
Cheers
Mike
Mike KilburnVice Chairman, HKBWSChairman, Conservation Committee