So with the help of this article http://www.timelordz.com/blog/?p=456 here's what I did.
From the command line I went into init 1 with the command:
Then the root password.
It'll shut down just about everything running and bring you to a prompt asking for the root password to enter maintenance mode. I again entered the root password then unmounted /home with:
Then I brought up the command line version of yast with
and brought up the partitioning tool and changed the size of the partition that gets mounted at /home (I brought it down by 1.5 GB). I've been caught in yast resizing partitions before with not entering the GB (or MB or TB) after the number. It took a while with this operation as the /home partition is fairly large but in the end I had 1.5 GB of unpartioned space.
Once done I went back into run level 5 by typing
The next step was to create a 1.5 GB partition and following the article in the link above I named it newboot. This was easy enough again using the partitioner in yast.
Once that was done again following along with the article I copied the contents of /boot into /newboot as root
cp --verbose --recursive /boot/* /newboot
The next step was to edit the /newboot/grub/menu.lst file. Trouble was that it wasn't immediately clear to me what to change it to. After a bit of trial and error (not really advisable when dealing with bootloaders I know!) I found that the newboot partition was at (hd0,9) in grubspeek (the old one was hd0,4). So, I got them changed over in menu.lst, crossed my fingers and typed 'reboot'.
When the thing booted I was quite pleased with myself until I realised that it was still booting from the old /boot partition. The fix was easy if a bit scary. I hadn't done the bit from the article where it says to:
grub> root (hd0,9)
grub> setup (hd0)
So again crossed my fingers and typed reboot.
This time it booted but didn't have the fancy grub menu that openSUSE comes with. I then saw that I hadn't modified the gfxmenu line. I changed that to match the (hd0,9) of the kernel image I was wanting to boot from and all was good....
Well, at least until I tried to run the kernel update with:
zypper -n up
It was still erroring out saying there wasn't enough space on /boot to install. So, it's trying to install to /boot rather than /newboot. Makes sense, the standard place for it is /boot. No developer has any idea that I've gone and told my machine to boot from /newboot. So, back into the yast partitioning tool. I changed the mount point of /boot to /oldboot and that of /newboot to /boot saved the changes and rebooted just for good measure.
When the system came back up it booted from hd0,9 as written to the master boot record when I ran the commands at the grub prompt before. This time however the partition at (hd0,9) was being mounted as /boot rather than /newboot. So again I ran the update:
zypper -n up
This time with all the pieces in place the new kernel installed on my new 1.5GB /boot partition.
So thank you Pete Eby for the article that pointed me in the right direction.