After trying to boot from a live CD image, pretty Esc frantically to get into the grub boot menu and trying to find boot parameters that would boot the VM into rescue mode, I finally found the answer…
In XenCenter, go to VM > Start/Shutdown > Start in Recovery Mode
Annoyingly that simple.
After a while of getting errors while trying to install an Ubuntu ISO to a new Ubuntu template VM, I got it working using the following URL for the install from web:
UPDATE: this URL also works – http://www.mirrorservice.org/sites/archive.ubuntu.com/ubuntu/
(The path they’re looking for is /dists/lucid/main/installer-i386/current/images/netboot/xen/vmlinuz)
Not sure why, but all of the mirrors I found apart from this had the wrong folder structure, and XenCenter wanted to find the ISO itself a few folders deep, rather than let me give it the exact location of the ISO to install.
Installing XenServer Tools is also easy. Put the tools into the virtual drive, mount it, navigate to it and into the Linux directory, pick your package (64 or 32 bit) and install:
mount /dev/cdrom /media/cdrom
dpkg -i xe-guest-utilities_5.6.100-651_i386.deb