Actually I worked out everything that was wrong. The places it didn't work that didn't need a password, it probably didn't work because I had left the wired internet configured that we used to talk between our computers, so if the gateway was 192.168.1.* then it tried to find it via the unplugged wired port.

And for the place that needed a password, I spent about fifteen minutes just now discovering that in fact I could use it, so long as I edited the right plain text file in the right way and ran the right command line program. Quite why I don't know, but meh. It works now.