Additional Addreeses, DNAT and FTP

Hi All
I'm having some trouble with my FTP server behind and Astaro ASG120, when i look at the logging on Filezilla all traffic seems to come from my static IP address rather than showing the actual IP of the user connecting which means i cant use autoban as it blocks all traffic after a few minutes :(. I've just had BT Infinity installed so now have 5 static IP addresses but BT give you a dynamic IP address as your primary address then map the 5 static IP's to your login credentials so I have set up the 5 static IP's as 'Additional Addresses' then setup a masquerade rule to send all traffic out of the first additional address, I then setup the following DNAT rule:

Traffic Source: Internet IPv4
Traffic Service: FTP
Traffic Destination: first additional address
NAT mode: DNAT
Destination: Internal FTP server address
Destination Service: FTP

Am i doing something wrong? This all worked fine before BT Infinity when my static IP was my primary address

Thanks in advance

Karl