After running a long test with the thermistor away from the TEC and the TEC between two heat sinks. So that the board was running at the current set limits. I found that the PCB heated up a lot, probably more heat than the TEC itself was producing. Due to the heat the op-amp system flipped, so that the it was no longer trying to cool, but was now heating.

The heat generated by the chip, suggests significant inefficiencies, this coupled with the cost of the chip itself ($45 from mouser, $20 for 2 on ebay), i've decided to work on a alternative.

I've chosen the Analog Devices ADN8831 TEC controller that works with external MOSFET H-bridges, so that I can use more recent, higher efficiency components. The controller is also priced at ~$10 for single units so is a more economic prospect.

AD have also provided guidance for using this controller for higher voltage TECs by simply using voltage level converters to connect with the H-Bridges (not required for my application).