Set Appliance State:

Set Multimeter to Resistance Set meter to measure resistance
Isolate Mains Power to appliance as per TOPs P4
CH Boiler Thermostat to Maximum
Remove the Orange wires from the CH Boiler Thermostat
Ensure Boiler is cool

Measurement:

Measure the resistance across the terminals of the CH thermostat.
Reconnect wires after test.