I vaugely recall something, but I would check this as I might have it wrong.
Put the sender in boiling water, fit a variable resistor inline and connect up to the gauge. As the water is boiling it should be around 100C and you should therefore see 100C on the gauge. If you don't see 100C adjust the variable resistor until you do. Disconnect it all and with a multimeter, measure the resistance. Once you have your resistance get a fixed resistor from RS or Maplins and then solder it inline on the back of the gauge. Run the test again to make sure that you are seeing 100C on your gauge and hopefully you should be ok.
Again, got to stress I might have picked this up as a method which doesn't work I just can't remember. It makes sense to me, but so do many things which don't work ;D
Andy