Set Appliance State:

Set Multimeter to Resistance Set meter to measure resistance
Isolate Mains Power to appliance as per TOPs P4
Ensure Boiler Temperature is hot enough to satisfy Thermostat
Remove Orange wires from CH Thermostat

Measurement:

Measure resistance across terminals of thermostat.
Reconnect wires after test.
Note:
Maximum flow temperature for CH is 82 degrees C.