Dave
What a very simple and brilliant idea. With a taut wire you should be able to get a very precise line.
Anybody got an old junior hacksaw frame they don't want?
I use the multi-strand picture hanging wire you can get from DIY shops. Very very strong. You can twist it to itself, so attacheding to two wooden handles is easy.
(Ops, should have read the second page - its been suggested already. The bit I used has burnt probably 100 rings and not yet broken).
The back of an old hacksaw blade works well - or turn the blade of a junior hacksaw backwards for a thinner line. Old credit cards don't sound like a good idea to me - they are PVC and I suspect will soften too easily. The brown PCB material is basically formica without the decorative layer