Hi Bob
Thanks for the swift reply. I continued searching through the topics after I asked the above question and found an earlier answer from you to someone else's query about a year ago. Although the question was slightly different, the answer seems to cover this instance too. My new internet serivce provider had forced my Extreme to automically flick into Bridge Mode. Thus all the Guest Network settings disappeared from Airport Utility.
What was strange however, was that the Extreme still seemed to be broadcasting the earlier Guest network that I had configured while using my old service provider. I wasn't actually able to try it (as I forgot the password long ago), but it appeared in the WiFi Network lists of all the iPhones etc in the house.
In order to diasable it I simply performed a factory reset on the base station, then re-instated all the other network settings to where they were. The ghost guest network has gone now.
In case this is of any interest to you I'm running OSX 10.6.8, with a 4th gen Extreme, and Airport Utility 5.6.1.
Thanks