The actual method of resetting the appliance after a fault code differs from the manufacturer’s instructions.
The makers instructions suggest the power / reset button (highlighted on the image) should be pressed for "1-3 seconds" or "several seconds" and then released. However if the button is held for more than 2 seconds this will attempt to turn the boiler off which does not reset the fault code.
To clarify the correct method of resetting the appliance is to press the power / reset button (highlighted on the image) for one second and then release.
If the boiler will not reset using the method above a hard reset may be required. To carry out a hard reset press the power / reset button and CH temperature + button simultaneously for one second then release.
If "normal" and "hard" reset methods fail to reset the appliance further investigation will be required and the PCB may need to be replaced.