Thanks Deepak. The info in your post helped me achieve a similar thing.
In my case the servers that needed updating were in a DMZ and only had port 80 open to the LAN. I was therefore very limited in what update services they could access.
So, much like your FTP instructions, I instead created a website on a server on the LAN, set up a scheduled job to FTP the NAI update area down to it and publish it. I used WinsCP to perform the synchronisation as WGET didn't do that very well.
I then created a new Autoupdate task in Viruscan console on each server to get its updates from this http server. All seems to work very well.