Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Turn Hot Water Tap On
Boiler On/Off switch Off
Remove and disconnect internal programmer
Gas On
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Observation:

Boiler On/Off switch On.
Reset lockout if necessary.
Does the Fan now run?
Refit programmer after test.