Thanks Shack..............the photo's fine if you KNOW how it works but not quite as good as a schematic to explain the mechanism.
The CSU......the bit that senses engine RPM........was mounted at the front of the engine and controlled the delivery of oil to the moving cylinder mounted in front of the front prop which it moved by means of helical gearing meshing with segments on each blade................absolutely bog-standard for hydromatic variable pitch props. The clever bit was transmitting the front prop movement to the rear one rotating in the opposite direction.
Thanks MReyn, I'll pop in to Vallance Byways next time I'm in the Gatwick area.