STOP PRESS! I think I may have solved it - sort of.
I was trying to do what worked for @DaveR and suddenly a dialog box popped up asking to allow Logitech to control things. I am pretty sure this came up during installation and I said yes. Anyway, the relevant box was unticked. After I ticked it, things began to work.
It says enter a “valid key combination”. For me that seems to mean there has to be a modifier key. I can’t get it to work with a single key press. Even trying to delete your selection is difficult because it then accepts that as the input! The workaround for me will be to change my SU shortcut to Ctrl-M because it doesn’t matter if I use the mouse button instead.