Point taken about others using other distros, Roy. My own main concern
was whether the various different Ubuntus would make a difference. Since
presumably the /etc and /boot folders are in completely different
partitions for the different installations?

I've just managed to do most of what I wanted, using a gui front-end
called grub-customizer
(ubuntuforums.org/showthread.php)
and running from the Natty installation (which was the latest one I'd
installed).

As for Kubuntu: I had thought that it was more than just the desktop
environment that varies between them, and I wanted the "full" Kubuntu
experience (since you're so enthusiastic about it :-) If as you imply
it's essentially just KDE then I could free up another 20G - at the cost
of doing another complete wipedown (when I've tried to adjust partition
sizes before I was unable to join up the freed spaces).

A related question (but should we start a new thread?): when I tried to
install Kubuntu to use my separate /home partition (and I believe you've
said since then that you shouldn't do that) it wouldn't start - got as
far as the login screen but then won't accept the login. However my
Natty installation has picked up all my /home docs and settings - I
think I inadvertently set it to mount /home though I had intended not
to. It looks like separate installations of Ubuntu versions /can/ use
the same /home, whereas different distros, even when it's just Kubuntu
vs Ubuntu, can't. Is that right? (Obviously if I just switch between KDE
and GNOME that wouldn't be a problem.)

Oh! (Lightbulb!) - That's why my attempt to try Fedora didn't get very
far. Maybe that would be a better way to use the freed 20GB.