Thanks! That got me part of the way there - but I'm not 100% sure it's progress.
So, here is how it was set -
# KEYBOARD CONFIGURATION FILE
# Consult the keyboard(5) manual page.
XKBMODEL="pc105"
XKBLAYOUT="us"
XKBVARIANT=""
XKBOPTIONS="lv3:ralt_switch,compose:lwin"
BACKSPACE="guess"
Below is the same file from an ubuntu-gnome box at my work (different hardware), which is also the same as the contents of that file on Ubuntu running on this same laptop as Siduction.
# KEYBOARD CONFIGURATION FILE
# Consult the keyboard(5) manual page.
XKBMODEL="pc105"
XKBLAYOUT="us"
XKBVARIANT=""
XKBOPTIONS=""
So I backed up the "bad" one, and changed /etc/default/keyboard to be the same as the one above.
I can now set my super key to open the overview using the dialog shown below (I'm 99% sure I tried this with the old file in place - and it completely ignored my attempts, but will swap them back and check to be sure). But as you can see, it doesn't regard it as my super key. It regards it as "multikey".
So - I'm still not sure why this is so, plus I can't use any other "super +
x" shortcuts unless I go through and change super to multikey everyplace...