Set Appliance State:
Gas On; Pilot On
Set Multimeter to Resistance 
Isolate Mains Power to appliance as per TOPs P4
Remove User PCB from boiler and test the CH Potentiometer on the back of the board
Remove User PCB from boiler and test the CH Potentiometer on the back of the board
Measurement:
Measure across left and middle terminals of potentiometer, resistance values obtained should be as follows:
Approximately 1.9 Kohms with potentiometer turned fully to Maximum.
Approximately 0 ohms with potentiometer turned fully to Minimum.
Then repeat the test between the middle and right terminals, the readings should be opposite to above.
0 ohms on Maximum and 1.9 Kohms on Minimum.
Replace user PCB after test.
Approximately 1.9 Kohms with potentiometer turned fully to Maximum.
Approximately 0 ohms with potentiometer turned fully to Minimum.
Then repeat the test between the middle and right terminals, the readings should be opposite to above.
0 ohms on Maximum and 1.9 Kohms on Minimum.
Replace user PCB after test.
Isolate as per TOPs P4
and refit as necessary