Hooray, WICD (Wireless Interface Connection Daemon) works and the laptop connects automatically when I boot it up again. It doesn't have the bug about losing my WPA password and it's also got a much more helpful interface than the supplied Network Manager. A similar situation to a lot of supplied Windows software, I suppose, other people have usually written much better tools for doing most jobs.
The only remaining problem I've got is that the desktop doesn't recognize the presence of the 1055 USB adapter on boot-up, until I unplug and replug it. I've rung the Voyager helpdesk and they've suggested ways of testing whether it's faulty.
Edit: as I was typing the above, I suddenly had an idea - the software for my old modem was still starting at boot time, presumably interfering with that for the 1055. So I've disabled that and all is now fine, both machines connect automatically when they boot up
To be fair, the manual for the 1055 did say uninstall your old modem but I didn't at the time in case I needed to use it again if I couldn't get wireless working.