Set Appliance State:

Gas Off
Isolate Mains Power to appliance as per TOPs P4
Turn Hot Water Tap On
Disconnect the ignition leads from the spark generator and make safe
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Observation:

Reset appliance as necessary, allow the boiler to
complete its self check and begin ignition sequence.
Does the display still flicker during sparking?