I have attempted to get to the bottom of the preseed file being ignored, and have failed to do so. I've submitted a bug report explaining what I tried, which wasn't a great deal because I don't really understand how debconf works. The addition of a file in /lib/live/config which adds user to a nominated group is a satisfactory workaround for now, so the bug isn't a show-stopper. Sorry not to be more help on this one. :( Nick/.