Probably the solenoid valve.
Solenoid valve comes as part of an assembly but you can buy it separately - saves a few quid.
You need to ease the duffer off the assembly with a screwdriver and fit the new one.
You'll get the assembly part number from the installation instructions - looking for the assembly on the web should lead you to a part number for the valve itself.
Shouldn't have to turn the water off but certainly the electricity.
Gordon
November 2010