Neil,
A couple of thoughts:
From your posting, it seems like you are trying to search an associated entity by setting up configurations in the top of this tab on the widget:
because you said the Operator is contains. But for references, the configuration should be made in the Ref search filters section. Maybe you could clarify a bit.
Also, because you are trying to search on Account, you may run into some security issues...the searching user may need rights to manage all users in order to be able to search. This is because, in the case of user entities, the settings in Project Security | User Roles tab override entity security settings.
Hope that helps,
Mike