Unable to re-configure Airport Express

I've got three Airport Expresses which connect wirelessly to my router (BT Homehub3). Two are used purely for Airplay, the third one also provides a wired connection to another device.

I recently added an iMac to the wireless network, and had to change the wireless security settings on the router - couldn't get the iMac to work with WPA, so switched to WEP. This meant that I had to re-confugure the AEs. Two were re-configured successfully, but the third one is causing problems, flooding the wireless network when it tries to connect. I have to unplug it, and reboot the router to get the wireless network up again.

I've tried a factory reset (switch off, hold reset button, switch power on, holding button until LED flashes), but as soon as I start Airport Utility, the device is recognised with the name that I gave it, rather than the Airport ID. I can configure it using Airport Utility 6 and an ethernet cable to my router, but as soon as it restarts (ethernet cable unplugged), it crashes my wireless network.

If I do a factory reset and configure it using my older iMac (Airport Utility 5), it shows as the Airport ID, but still crashes my wireless network. Also, Airport Utility 5 doesn't give me the option to select which wireless network I want the AE to join. It automatically picks the wrong network!

How can I do a proper factory reset, and also make Airport Utility "forget" the device, so that I can configure it from scratch?