I have a 2007 Noma thermostat and it is set for 17.5 Celsius. It is now 23 C in my house and the boiler is still running. What do I do other than check the wiring?

If it gets to hot turn boiler off at main disconnect. You can check to make sure it is not the thermostat by disconnecting one of the wires for the thermostat at the boiler. If the boiler shuts off after thermostat wire removal the problem is in the thermostat wiring being shorted OR the thermostat itself.