Ian,
My guess would be that if you connected a supply capable of going down to 0V thru a ~33ohm series resistor to sender wire, you'd find 0V=needle @120C and slightly less than 2V=needle @50C. (The 1.5V battery in series with the supply as mentioned earlier does work. I've used it before).
If you wanted a larger usable voltage swing to control this, then connect the sender to a 33ohm directly to ground (this becomes the ballast resistor). Then connect the power supply to the sender wire as well, but through a larger value series resistor. You could play around using a 1K pot to determine what this value should be. Key first is power supply that can get to 0V
