I have a working Ubuntu partition on one of my hard drives, however grub is not installed. What I would like is a 'boot disk' which allows me to boot this partition. Does such a thing exist? I'm pretty sure I remember using something of the sort in the distant past...
The Ubuntu Install Disk is a 'Boot Disk' in the sense that you can get into a working environment to fix whatever is causing the system to not boot. Ubuntu can be installed from Optical media by burning the ISOs to a disk in any operating system. Live USB installers can be made from Ubuntu with the Startup Disk Creator program and an ISO image. USB installers can be made from Windows by following the guide here, called the Universal USB Installer.
If you want to actually be in the environment of your brokenn installation, you can press 'c' for Command Line during the Grub menu and can manually setup a new entry or press 'e' for edit on any of the existing entries and change it so it will boot again.