Set Appliance State:

Mains Power Off
Set Multimeter to AC Volts Set meter to measure AC Volts
DHW demand from programmer
DHW temperature selector to high setting
Ensure hot water storage vessel is calling for heat
Boiler On/Off switch Off
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

Boiler On/Off switch On.
Reset lockout if necessary.
Measure voltage across the flame sense electrode lead at the flame sense electrode and a suitable earth point.
Expected voltage, approximately 55 volts AC.
Note:
It can be difficult to test inside the rubber cover on the electrode, take care to ensure good contact is made.