Tachometer (Rev Counter) not working

Common fault
Diesel Tachometers rarely work on older cars. Replacing the sensor which is a one screw job once you have located it (either at the bottom of the bellhousing or behind the hydraulic pump) will almost certainly fix it. The sensor itself measures 270 to 330 ohms when healthy.
Any higher and it has packed up.

Note from Jon-

The sensor attaches to the engine block and can be found if you look above the diff housing on the gearbox where the RH driveshaft goes in. The sensor is attached with a 10mm headed bolt. Follow the wire back, and it terminates in a brown connection with 3 pins, between the battery and the bulkhead, near where the main wiring loom goes through the bulkhead.

The sensor itself attached to the block with a 10mm headed bolt, its a spanner job only as its quite tight.