Thanks for the tips. I ran 'boot-repair', but it didn't change anything.
I assume that this means that the previously bootable installation of Fedora on /dev/sda5 is no longer recognizable to
the grub installer. I can mount this partition and read files from Mint on /dev/sda1.
I am rather convinced that something in the Mint 13 installation into one partition 'disables' the bootable installation of Fedora on a different partition. I've done this sort of multiboot installation dozens of times with OpenSuse, Ubuntu, Fedora etc etc, and in the last few months this problem has happened at least twice when I have installed Mint 13.
I carefully select which partitions to install into (/ and /home in different partitions). I just leave the swap partitions alone - is that part of the problem ? I assume that if the installer knows which partitions are swap it will use one of them.
I also inadvertently switch between 64- and 32-bit installs. I usually use 64-bit, but the most recent install of Lint 13 was from a magazine multi-distro and the Mint was 32-bit.
I can recover by reinstalling Fedora where it was, but I'd prefer not to have to, natch.
Is this a bug, or should I do something differently next time ?