After installing Fedora, I cannot bring Ubuntu back

a Fedora command-line would probably have been the best one to type the commands from if you're there already.

Grub2 doesn't use /boot/grub/menu.lst. Grub2 configs are located in the dir /etc/grub and in the file /etc/default/grub.

So, to change your configuration (like the timeout and the boot options), you need to change the file: /etc/default/grub
And to add splashimages and kernels and such, you need to modify the files in: /etc/grub

Be aware that Fedora11 Grub2 needs a ext3 /boot partition (Boot cannot be in an ext4 formatted / partition)
http://docs.fedoraproject.org/install-guide/f11/en-US/html/s2-diskpartrecommend-x86.html

Have you check /boot/grub/grub.cfg?
Is your Ubuntu Jaunty /boot directory in an ext3 partition?

Read some Fedora's documentation before do any change, and make some backups:
http://docs.fedoraproject.org/install-guide/f11/en-US/html/ap-admin-options.html
http://docs.fedoraproject.org/install-guide/f11/en-US/html/ch-beginninginstallation.html#sn-boot-menu

If none of this help maybe you might be interesting in "howto install grub2 in Jaunty":
http://gastly.iblogger.org/how-to-install-grub2-on-ubuntu-jaunty/

To add a new operating system entry in the menu, you must create a new file in / etc / grub.d. For example, to add an entry to boot Windows XP you can create a 20_windows_xp file in / etc / grub.d with the following lines:

    #! /bin/sh -e

    cat << EOF
    menuentry "Microsoft Windows XP Professional" {
    set root=(hd0,1)
    chainloader +1
    }
    EOF

GRUB Legacy partitions are numbered from 0, which were always a number less than those indicated by fdisk in Linux, and this has brought us headaches more than one. With GRUB2, this has change and numbers are like in fdisk.
So if before (in your grub menu.lst) was (hd0, 0) now with GRUB2 is (hd0, 1)

Finally give permissions to the script and re-generate the configuration based on these templates:

# chmod +x /etc/grub.d/20_windows_xp
# update-grub

And reboot.

**Read the FEDORA documentation before any change.
Hope that helps.

Sorry, yes - of course. However, grub couldn't find /boot/grub/stage1 probably because I installed grub2 in jaunty.

I tried installing grub2 from livecd - but couldn't work out how to manage the task.

Hmmm filling out this reply, I am offered only one option to click 'This Solved My Problem' - what's the story there? can't I just post a reply?

olz okay, this time I can 'add a comment' or 'still need an answer'.

I'm sorry, but having downloaded '9.10' cd I just went ahead and installed over my former root partition.

find returned an error (25) I think, and no operating system was found. My internet cut out, and I decided to go ahead instead.

I guess I'll meet a whole new world of bugs to help out with in Karmic - though so far I'm rather impressed by the boot time - around 5 seconds faster after selecting the grub option (maybe 15 seconds to load, add 20 seconds for the BIOS).

Thanks to pablitofuerte for a very comprehensive answer - this may prove excellent reference material for the future.

Alpha release! :) Great work there, thanks Ben. Err, you've somehow given me credit for this by mistake but i'm not complaining ;)
Regards from Tom :)