Set Appliance State:

Mains Power Off
Gas Off
Set Multimeter to AC Voltage Set meter to measure AC Volts
Summer/Winter Switch set to Summer.
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

Boiler On/Off Switch set to ON.
Measure voltage between N and pin 31, bottom right of PCB.
CAUTION : Heat Sink is Very HOT on R560.