And I thought I was generally quite good at obeying the maxim: thou shalt not fiddle with a computer when thou art tired, at the end of a long day. First time I've made that mistake in a while, and you're absolutely right Digerati, the touchpad had been switched off. Not sure how though, as I hadn't knowingly done it, and the owner claims not to have done it either.
Anyway, got the very latest drivers on there (100MB for a touchpad!), made sure the Fn key plus Mouse key were set to allow the mouse to operate, and everything's moving, scrolling and zooming again.
Never really seen the point of turning the touchpad off, when it's usually deactivated by an external mouse being plugged in, AFAIK.
Thanks again for the help for this dunce.
Regards, Jon.