Consensus is that I don't need the interrupt as temperature control isn't really time critical, and the RS232 for reading the temperature is quite slow anyway.
I am unsure at this stage of how fast the volume I am controlling will heat up/cool down. I will need to run some tests first. If it turns out my code does not loop fast enough for it to effectively control the temp, I will look at using the interrupt.
Thanks for all the help, you guys were great.