Observation:

Manufacturer instructions state this boiler should be fitted with a permanent mains supply.
British / Scottish Gas branded boilers provide an alternative if no permanent live is available, please refer to MIs and ensure these instructions have been carried out correctly.
An appliance fitted without a permanent mains supply will not be able to provide
a pump overrun after a demand, this may cause overheat or noise issues.
You can still use this model to fault find if you wish, but if you are asked to
test the pump overrun operation then this is the reason that it will not overrun.
CLICK HERE TO CONTINUE FAULT FINDING.