I thought I'd post this just in case it helps anyone who is in a similar position, as I wasted hours Googling around before I finally guessed the answer.
I started with a Multiboot of XP, Mint 9 and VectorLinux with LILO in the MBR.
I wanted to install Mint 12 (from DVD) to overwrite Mint 9 with Grub in the MBR (I realised that would make it impossible to load Vector but I didn't really need that anyway).
Everything went well. I told the installation which partition to use and reused the Mint 9 swap partition. When I rebooted though, all I got was a "grub>" prompt.
I was able to boot into Mint 12 manually by issuing the appropriate CLI commands. I then ran "update-grub" and got a load of syntax errors. I purged and reinstalled Grub but nothing made any difference.
Finally, I wondered if the Vector partition was confusing Grub, so I went into the Disk utility and deleted that partition. Then I reran "update-grub" and this time got no errors. Rebooted and there was the menu .