Set Appliance State:

Mains Power Off
Gas Off
Set Multimeter to Resistance Set meter to measure resistance
Ensure secondary heat exchanger is cool and requires pre-heat (this can be achieved by running the DHW with the gas to the appliance turned off)
Remove the spaded red wires from pre-heat thermostat (located on flow between diverter valve and secondary heat exchanger

Measurement:

Measure resistance across the exposed terminals of the pre-heat thermostat.