Rob,

I add a user, and all seems well (including the creation of the user's directories, etc), but the new user does not appear either in the user list in the accounts preferences pane, or when I use the login screen! I've also
tried rebooting with no effect.

Are you Admin or Root whilst adding users?

Admin, but this has never been a problem before.

And in reply to Josh:

So, Andrew, if you're not as lazy as me, maybe try reinstalling 10.4?

I am pretty lazy... However, if all else fails I'll try this in a few days time.

Cheers,

Andrew