Your boiler's outside thermometer has developed a fault, causing the system to 'think' it is 0°C outside and adjust your heating accordingly.

Controls according to 0°C outside temperature - Outside temperature sensor shorted out

What causes this fault?

This fault occurs when the electrical connection to the external temperature probe is broken or short-circuited. The most common reasons are moisture getting into the sensor housing, damage to the cable running along the outside wall, or the internal component simply failing over time.