Set Appliance State:

Gas On; Pilot On
Turn Hot Water Tap On
Isolate Mains Power to appliance as per TOPs P4
Boiler Operating Switch in HW position
Remove Yellow wire(s) from Modureg (1) and make safe
WARNING! Mains voltage is expected during test, follow TOPs P4 live testing procedure
Mains Power On

Observation:

Does the burner cross-light at low flame from the pilot and stay at low flame?