I had pointed "Contexts" to a security group, and left "Object Class" blank.
To get it working:
"Contexts" was pointed to the users OU
ou=users,ou=uk,dc=contoso,dc=local
"Object Class" was specified as the security group that my Moodle users were in (full query string below)
((&(objectCategory=Person)(sAMAccountName=*)(memberOf=cn=moodle_users,ou=security groups,ou=groups,ou=uk,dc=contoso,dc=local))