Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Remove external connections to BL terminals, leave in a safe place
Fit a wire link between BL terminals
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Observation:

Reset appliance, press and hold the reset button until the chimney
sweep button led flashes rapidly, then release the reset button.
Does the boiler reset?