Set Appliance State:

Set Multimeter to Resistance Set meter to measure resistance
Isolate Mains Power to appliance as per TOPs P4

Measurement:

Remove both the violet spaded wires from the DHW Pre-heat thermostat, and measure the resistance across the thermostat.
Replace wires after test.
DHW Pre-Heat thermostat is the upper most of the
two thermostats situated below the diverter valve.