Set Appliance State:
Set Multimeter to Resistance 
Isolate Mains Power to appliance as per TOPs P4
Depending on which gas valve is fitted unscrew cover and remove brown and blue leads or unscrew and remove plug from the top gas valve solenoid
Depending on which gas valve is fitted unscrew cover and remove brown and blue leads or unscrew and remove plug from the top gas valve solenoid
Measurement:
Measure resistance between the exposed terminals of the top solenoid.
Expected resistance for both types of gas valve is approx 5.1kOhms.
Expected resistance for both types of gas valve is approx 5.1kOhms.
Isolate as per TOPs P4
and refit as necessary