Temp knob does not reduce temp, however I turn it

My fellow stag electric has a problem after 2 years of use: the temperature and turn on off dial does not increase the temperature. Even if moving the dial clockwise the temp will only decrease. I have to take the cattle off the mains, turn it on, the temperature then shows 100C and I have to turn the knob counterclockwise to reduce it to my working temp of 93C