Thanks for your help with this PeterH, this is the page that first alerted me to Arduino and our conv. has re-whetted my appetite for an inexpensive route - maybe I should get a Arduino reference book to fill in some blanks such as interfacing it...
In the mean time I also dug up reference to the Windows Raw Input API that looks interesting and I have some experience that should help get started with it. In the particular example scenario outlined, two mouse were considered and so maybe I could get that working in Excel for example where I have VBA experience.
As ever I'll keep my ears to the ground but continue to look around - cheers 2Tricky