I didn't know that two females died at Chester, when did this happen? I think that there is a problem with males killing their mates in captivity. I would imagine that if Chester were offered a female from another zoo, then they'd take it.
Chester had amazing success with Blue eyed cockatoos and they were on show, so it must be just a problem with philippine cockatoos as a species.
Regarding BoPs at Chester, couldn't they get a female from where Sebbe got his? They are on sale from private breeders, but couldn't the zoo buy some?