Set Appliance State:
Set Multimeter to Resistance 
Isolate Mains Power to appliance as per TOPs P4
Remove plug X1 from the PCB
Remove plug X1 from the PCB
Measurement:
Test 1:
Measure the resistance across the brown wire (potentiometer wiper, middle terminal) and the blue wire at the CH potentiometer (boiler thermostat).
With the potentiometer turned down fully (anti clockwise), the resistance should be short circuit or low resistance.
Turn the potentiometer to maximum (clockwise) and observe the resistance, resistance should increase to a maximum of approximately 10 kOhms.
Test 2:
Measure the resistance across the brown wire (potentiometer wiper, middle terminal) and the black wire at the CH potentiometer (boiler thermostat).
The resistance should be short circuit or low resistance (thermostat at maximum).
Turn the potentiometer to minimum (anti-clockwise) and observe the resistance, resistance should increase to a maximum of approximately 10 kOhms.
Measure the resistance across the brown wire (potentiometer wiper, middle terminal) and the blue wire at the CH potentiometer (boiler thermostat).
With the potentiometer turned down fully (anti clockwise), the resistance should be short circuit or low resistance.
Turn the potentiometer to maximum (clockwise) and observe the resistance, resistance should increase to a maximum of approximately 10 kOhms.
Test 2:
Measure the resistance across the brown wire (potentiometer wiper, middle terminal) and the black wire at the CH potentiometer (boiler thermostat).
The resistance should be short circuit or low resistance (thermostat at maximum).
Turn the potentiometer to minimum (anti-clockwise) and observe the resistance, resistance should increase to a maximum of approximately 10 kOhms.
Isolate as per TOPs P4
and refit as necessary