ref: baxi solo wm70/4 - overheat thermostat?

Having problem resetting overheat thermostat. When pressing in plunger to reset will not stay in and can feel resistance every time depressed (will not click in and stay in). If I hold it in boiler will fire up but as soon as let go stops. Have drained system and checked pump, eveything is working OK, no airlocks. Would replacing with new thermostat cure problem?.