Thanks for your reply.
I think so. You need to edit, as root, the file /etc/X11/xorg.conf, this stanza:
Hmmm. This is what my xorg.conf file looks like in Kubuntu
Section "InputDevice"
Identifier "Configured Mouse"
Driver "mouse"
Option "CorePointer"
Option "Device" "/dev/input/mice"
Option "Protocol" "ExplorerPS/2"
Option "Emulate3Buttons" "false"
Option "Buttons" "7"
Option "ButtonMapping" "1 2 3 6 7"
Option "ZAxisMapping" "4 5"
Option "Resolution" "100"
EndSection
I wonder, and will probably try to set it the same as Kubuntu to see if that does anything.