I'm blogging this because I can't believe the fix and I'll know i'll forget it. In short my Airport Extreme was only allowing one mac to connect at any one time. Which is seriously aggravating for me as I need files on both my macs. After spending all my lunch reseting the base unit and trying a host of different configs which all ended up with only one machine connecting I found this thred which said that you need to plug the second machine via ethernet into the base station and then unplug it. THATS IT!!! Viola. Problem solved. Thanks Apple
I connected an Ethernet cable to my AirPort Express, and to the Mac Mini, then ran the Network Setup Assistant - System Prefs.> Network > Assist Me > Assistant. The Mini didn't connect to my network. I unplugged the Ethernet cable, and ran the Network Setup Assistant again, and while I was doing so, the Mini connected automatically.Evidently, the mere act of plugging and unplugging the Ethernet cable was enough to refresh the whole thing.When I plugged it in, the Mini saw a "new" network and tried to connect to it. For whatever reason it didn't, but when I unplugged the cable, the Airport Express "refreshed" the IP address, and the Mini went back into 'wireless search mode'. It found my network, connected to it automatically, then I was able to open Safari and start downloading por ... ahem, I mean, um, reading poetry blogs.