Secure FTP woes using iptables - Expert help needed

We have a requirement to connect to an external Secure FTP server. However,
I am having the same old problem of being able to created a data connect.

Did have problems with normal ftp active data connection but used

modprobe -dv ip_conntrack_ftp ports=21
modprobe -dv ip_nat_ftp ports=21

and it resolved my issues.

External Secure FTP remote server is running on 10021 and 10020 (data). I
am able to log in ok but when I issed an "ls" command the server returns
"Cannot bind data connection"

I have modifed the above to read

modprobe -dv ip_conntrack_ftp ports=21,10021
modprobe -dv ip_nat_ftp ports=21,10021

but i am still unable to created a data connection.

My iptables rules are as follows

eth1 = Internet interface
eth0 = Lan inteface

# Static one-to-one nat from <internal client> to <external-ip>

iptables -A PREROUTING -d <external-ip> -i eth1 -p tcp -m tcp --dport
10020:10026 -j DNAT --to-destination <intnernal client>
iptables -A POSTROUTING -s <internal client> -o eth1 -j SNAT --to-source
217.<external-ip>

iptables -A INPUT -p tcp -m tcp --dport 10020:10026 -j ACCEPT
iptables -A FORWARD -p tcp -m tcp --dport 10020:10026 -j ACCEPT

note: you may have noticed that ports 10020 - 10026 have been forwarded -
this is what the remote server administrator advised.

What else am I missing.

Help my anyone, you're my only hope.


Regards

Sam