Set Appliance State:

Mains Power Off
Gas Off
Set Multimeter to DC Volts Set meter to measure DC Volts
Turn Hot Water Tap On
DHW temperature control to maximum
CH temperature control to minimum
WARNING! Mains voltage is expected during test, follow TOPs P4 live testing procedure
Mains Power On

Measurement:

Allow fan to run and ignition sequence to begin, once the sparking has started, measure the voltage across the connections of the modulating solenoid.
Expected voltage is approximately 10 Volts DC.