Bill, yes I'd seen your work... it was that that got me thinking...

Well I just dismantled the 6" one (its the same type as the 8 and 12) and its the same inside as Geoff's. I also discovered that you can get an extra 20mm past the marked scale. The reading heads are identical. Has anyone dismantled the actual scale bit to look inside? Similarly, has anyone found a source of the scale elements on their own? I found a firm selling the reading heads on their own.

Bill: I like your idea of grounding the scale and generating a -1.5v supply by offsetting the PIC supply. I think a simple -ve charge pump would do the job as well.