Set Appliance State:

DHW demand from programmer
DHW temperature selector to high setting
Ensure hot water storage vessel is calling for heat
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.
Allow boiler to fire under DHW demand.
Does the cylinder heat satisfactorily with no kettling from the appliance?