Thanks for the quick reply. Yes, I did choose the right model under the software installation. Very carefully!
The trouble is that Windows 7 itself seems to recognise the mouse as an "Intellimouse Web", and installs the drivers it wants to. It does this whether I install the software before or after plugging the mouse in! Neither makes a difference. (And I have rebooted between each installation / uninstallation). I'm confused.
The Model type is simply 'Intellimouse Optical' - (Product ID 52195 - 576 -8429023 - 00000). I know I haven't made a mistake here, because in Control Panel>Mouse there are pictures of all the different types as you try to choose the button settings. I'm choosing the right mouse for my hardware - but not for what Win 7 recognises it as!