Your boiler has shut down because the internal exhaust gases are getting too hot, which is usually caused by a circulation problem or a buildup of debris inside the main heating component.

Flue overheat - check combustion and flue temperature vs. return temperature (Installer numbers 7 and 3 above). The flue temperature should be no more than 10C above return temperature. Possible heat exchanger fault

What causes this fault?

This fault occurs when the internal temperature of the exhaust gases becomes dangerously high, triggering a safety shutdown to protect the appliance. The most common reason is that heat isn't being transferred effectively into your radiators, often due to a buildup of dirt or debris inside the main heat exchanger. It can also be caused by a lack of proper water flow through the boiler, which prevents the system from cooling itself down while firing.