Thanks for the suggestion. I did try, but the problem seems to be that "untraining" me from using my thumb on the left-clicks is very hard, so I'm thinking I need to try something completely different.
I went out this week and picked up a Wacom Bamboo Pen and Touch, but that wasn't the answer. Setting it up was onerous, then problems ensued with getting the pen to behave (it probably wasn't correctly configured) and I finally gave up. No doubt others have found it a wonderful device, but I found Don Norman's observations on this sort of thing being less to do with human error (as it's likely, if I'd invested lots more time, that it would have worked) and more to do with poor design came to mind (in configuring, anyway).
So, still got a sore thumb and wrist, and still looking for a solution!