Suspecting an error in the MBR, we booted again from the FC3 CD1 and asked for 'Update'.
This took us through the dialogue in which the installer said that it didn't recognize the boot loader and would we like to install GRUB. We answered yes, and that we wanted it in the MBR.
Then, in some insane fit, the installation script tells us that because we have nothing to update it is going to exit without reinstalling the boot loader.
I recall inflexible behaviour like this from older releases of RH.
How is one supposed to be able to reinstall GRUB into the MBR without redoing the entire FC3 installation.
Thanks