I assumed ZTL was correct in the ssp. ID but apparently not. Could I ask how you know it's of the mainland ssp.? I have images of the other Moustached parakeet at the zoo so perhaps that one is Javan?
@amur leopard There are two groups of Moustached Parakeets, which may be separate species. The alexandri group of Java and surrounding islands is monomorphic - both male and female birds have entirely red bills. The fasciata group is sexually-dimorphic - the male has a red upper mandible and black lower mandible (as in your photo), whereas the female has a fully black bill.
@Chlidonias Ah great thanks a lot; although a shame that it isn't Javan. I wonder whether the zoo did have Javan to begin with and subsequently brought in new stock or it never had Javan to begin with.