After much searching I found that the following article seems to be the best update path for 10.5.5->10.5.6.
The biggest problem is that I would need a USB keyboard.
Eventually I figured I could use the on-screen keyboard (Preferences something),
but when I did the update I lost the screen again, the old blue screen and mouse curson. It seems that is something a bit unique to my computer, and maybe one or two others. I revived my old advice put on forums.msiwind.net and here to use the terminal to fix the drivers - this time the kexts were already on the HDD though. But of course the keyboard had stopped working in the terminal too.
Eventually I found a USB keyboard and fixed the display drivers as before, then ran the package from the forums to fix the keyboard and trackpad. Weirdly, when I rebooted the Ctrl key was stuck down until I pressed it again - making clicking anything do weird things.