Set Appliance State:

Mains Power Off
Gas Off
Set Multimeter to DC Volts Set meter to measure DC Volts
Turn Hot Water Tap On
Ensure DHW is cool (this can be achieved by running the DHW with the gas to the appliance turned off)
DHW temperature set to maximum position
Gas On
Mains Power On

Measurement:

Allow fan to run and boiler to fire.
Measure voltage at the terminals of the modulating solenoid (grey wires).