H, thought I would write in to let you know that this problem has been resolved. After a discussion with the technical support guys at Smarter Mail, they helped pin point the problem to my firewall.
On further investigation, I found out that the http filters were blocking the .AXD extension, so a quick change to this setting and everything is now ok.
This explains why under any other port than port 80, this was working fine.
Thanks to everyone for taking the time to offer assistance and especially Daniel and the team at Smarter Mail for their wisdom!
Graham.