Faulty diverter valve or motor ?/?

Worcester Bosch Greenstar HE plus boiler, when the heating is not on we can get hot water but as soon as we have the heating on we can't get hotwater ?? We can manually move the motor on the 3 waydiverter valve to get hot water so I'm not sure if the diverter valve or it's motor is faulty?? It's very easy to remove the valve motor but once off the boiler how can I test it ? Any help would be greatly apreciated.

Darren
January 2010