Clive,
The original Holden sender is an on/off switch - it's normally "off", but turns "on" once you get to the "oh crap - too hot!" temperature. It won't drive a temperature gauge. To run the gauge, you need a sender that is always "on" - at low temp it is "a little bit on", and as things get hotter it gets "a little more on". Give the VDO guys a yell - they should be able to supply the right sender. You will need to think about where you are going to mount the sender though:
replace the original sender (then you get no idiot light),
tee into the original sender hole (then you get crappy temperature readings as the sender has no flow going past it),
use the second hole in the cylinder head (only good if you have a later FB/EK/EJ grey motor head),
use the spare hole in the thermostat housing (if you have an FX/FJ/FE/FC thermostat housing),
tap a new hole into the thermostat housing (if you have a later FB/EK/EJ thermostat housing),
try to find a sender that inserts under the radiator hose (similar to the Davies Craig thermofan senders).
Cheers,
Harv.