You could turn the interface into an abstract class, and provide default implementation for the methods as appropriate.
Update: I see, the multiple inheritance closes out changing the interface into an abstract class... in this case I would do the same as you. If the default implementation of the method(s) is not state-dependent, the best place for them is indeed in a static utility class. However, if there is state involved, I would consider object composition, which could even end up as something like a Decorator.
