Gregz:
I'm quite sure the temperature sensors in my ovens are thermistors. I don't think they'd use thermocouples because thermocouple voltages gradually diminish with use. This is why the thermocouple is the prime suspect if your water heater or furnace pilot light keeps going out. Thermistor resistance doesn't change that way, and so that predictability would make a thermistor better suited for use as a temperature sensor than a thermocouple.

WRP:
My sister was upset when she used an inexpensive supermarket oven thermometer to check the accuracy of the temperature in a brand new stove she had purchased just about two years ago now. It was off by at least 20 degrees; probably more. I borrowed her my digital oven thermometer, and she clipped the probe to the middle of the oven rack as the users manual for the digital thermometer recommended, but she was still getting readings that were considerably different than those displayed on the stove's console. It wasn't until she clipped the probe to the thermistor inside the oven that she got agreement between the two digital displays as to what the oven temperature was at any given time. But, in that case, she was really using my oven thermometer to measure the temperature of the oven's thermistor. But, that shows that the oven's thermistor isn't reading the temperature in the middle of the oven (where the food generally is), but in the top left rear corner of the oven, where the thermistor is, and she found a lot of difference in temperature between those locations.