Hi Eamon,
FTP uses two TCP Ports, one for the control session and a second one for data.
In your scenario, if the client is set up to make a 'passive open', the session will fail. Passive mode causes the server to send data to client containing the address and port number which the client should use to establish the data connection. This data does get translated only correct if the whole connection runs over the same NAT-router.
kind regards Peter