Baxi Solo 2 Gas Boiler 11 years old?

My Baxi Solo 2 Gas boiler has the following problem:

When the main power switch to the programmer is ON, and with all the programmer settings to OFF, the boiler still comes on and will not turn off with the room thermostat. It just appears to get hotter and hotter. Can you tell me if the likely cause is the thermostat sensor or the thermostat overheat sensor. Thankyou.