iBook keeps forgetting WiFi settings

I managed to break my wireless connection a few weeks ago when I was trying to extract the WEP key from the iBook to put into another iBook. Since that time I've not been able to persuade my iBook to remember the WEP key it quite happily used before.

I have an ad-hoc WiFi network, my Linux laptop connects fine all day long. When I got the iBook, it too worked fine, all day long. It would automatically connect on boot (having given it the WEP key). Now, the only way it will connect is if I select the Airport icon from the menu-bar, select "other" from the menu, choose my access point (usually the only one on the list but from time to time someone over the road switches theirs on), select the encryption type and type in the WEP key.

I don't mind this too much but my wife is the main user of the iBook and she's not the most technically literate - it doesn't help that the WEP key is 26 characters long, and hex, so not a memorable phrase.

It would be most helpful to go back to the original situation where on boot it would automatically connect. I have used the System Preferences/Network tool and put the correct details in there many times:
By default join: a specified network
Network: [my wlan]
Password: [26 char WEP key]

Clicking Apply Now chops the password down to 8 chars, but this could just be a security thing, not giving snoopers a clue to the size of the password.

I'm out of ideas. I, to a certain extent, hope I've been dumb and missed something, something that'll be pointed out to me and enable me to fix it easily.

Thank you for any help, suggestions or pointers you can give. Sorry for the long-winded post but brevity seems to have escaped me today, I'll try harder next time.