Eoin,
Here is a fundamental pseudo code description of your problem.
All the elements of this have been discussed in depth and recently on the forum. I don't know the SFR and operation of the ADC in the ADUC812.
Steve

Start:
READ ADC zero ;always an important step.
READ ADC thermometer.
Subtract readings of thermometer and zero..
using value above, Do lookup table for temperature.
Transmit value to serial port
OR LCD.