Set Appliance State:
Gas Off
Set Multimeter to Resistance 
Isolate Mains Power to appliance as per TOPs P4
Remove two outer screws, securing Boiler Control Thermostat to electrical box.
Remove two outer screws, securing Boiler Control Thermostat to electrical box.
Measurement:
Turn Boiler Control Thermostat fully anticlockwise.
Measure open circuit between the Black + Blue wires.
Turn Boiler Control Thermostat fully clockwise.
Measure short circuit between the Black + Blue wires.
Measure open circuit between the Black + Blue wires.
Turn Boiler Control Thermostat fully clockwise.
Measure short circuit between the Black + Blue wires.
Isolate as per TOPs P4
and refit as necessary