bottom oven on my AEG D5701-4M Competence fails to cut out at temperature?

My bottom oven heater element seems to carry on heating even after i have switched it off. i have to turn it off at the mains to get it to shut down. i have replaced the switch which made no difference and have had the thermostat probe checked out by some pyrometry guys and it looks ok. so would appear that a switch is not tripping when the correct temperature is achieved. where is this switch what does it look like? Is this the correct solution? Anyone heard of a similar fault?