Set Appliance State:

Set Multimeter to DC Volts Set meter to measure DC Volts
CH Demand
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

Measure the voltage between each of the wires, in turn at the terminal block at the bottom of the boiler to the left side of the resistor marked R24 on the PCB.
Expected voltage approximately 35V DC.
Is the voltage present on neither, one, or both wires?