Cheers Salteh 
Unfortunately the side scroll button function is not programmable. :disgust:
I sent Logitech a mail, and their reply was:-
Thank you for contacting Logitech’s Electronic Technical Support.
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I understand that you want to know whether it is possible to assign a different function or keystroke to the horizontal scrolling buttons of the MX1000 mouse.
Unfortunately, it is not possible to program the horizontal scrolling buttons of the MX1000 mouse. These buttons are not programmable. These buttons will work only for horizontal scrolling.
I apologize for any inconvenience this may cause to you.
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Bugger springs to mind.
Still it’s a nice smooth ride, accurate to the point of being silly and for a cordless mouse responsive and not at all laggy.