I installed Ubuntu on the spare 80G partition and used the alternative installer CD available Here which let me set up the disk with LVM. (Note : make sure that there is a separate /boot partition because grub has problems with reading a LVM disk). Once the disk partitioning was complete the rest of the instalation was pretty standard, although I installed grub into the new partition rather than overwriting the gentoo boot loader.
Once the Ubuntu installation was running I could mount the gentoo disk within Ubuntu and copy everything I needed into the new install (or as much as I could get into the space available).
Since this reduced the size of the gentoo install, I could then use the Partition manager software GParted to resize the gentoo disk and create a new partition in the resulting free space on the old disk. This takes a long time, in this case about 2 hours, so don’t worry that it seems to be not doing much for long periods of time.
I then prepared this new partition for use by the LVM manager and added it to the new Ubuntu system. This can be done without having to unmount the root file system on the new install.
Doing this I’ve been able to transfer everything onto my new install without having to back everything up and managed to keep the gentoo system available in case there are any problems in near the future. Once I’ve used the new system for a few months I’ll be able to junk the old system and recover the disk space by adding it to the LVM disk.
I should say that I keep all my important stuff like source code in a subversion repository held on a work server, and all the photos and videos of the family are backed up. If they weren’t I don’t know if I’d have been quite so cavalier.