Just altered my Xbox settings, so that it now uses DHCP rather than a fixed IP.
Been having problems all day today getting the Xbox to see the network in XBMP, but changed it to use DHCP restarted the Xbox and found the network straight away. This may just be a coincidence because as you mentioned restarting usually fixed it, just wouldn't today until the change.
The rest of my network uses DHCP, just always gave my Xbox a static IP, so I always new the address to FTP to. I have managed to get my ADSL router (my DHCP server) to always assign the same IP address to my Xbox, so acts as if it is a static IP address.
The test will be tomorrow when I start my Xbox, hopefully it will work 1st time. If not back to the drawing board.
Will post tomorrow to let you know if it worked or not, fingers crossed!