Despues de poner el mensaje, he visto que la solución sería esta:
Código SQL
[-]SELECT distinct
"INFORMES"."n_informe" AS informe,
"INFORMES"."n_propietario" AS n_propietario,
(SELECT "PERSONAL"."c_nombre" FROM personal WHERE
"INFORMES"."n_propietario" = "PERSONAL"."n_codigo") AS c_propietario,
"INFORMES"."n_envia" AS n_envia,
(SELECT "PERSONAL"."c_nombre" FROM personal WHERE
"INFORMES"."n_envia" = "PERSONAL"."n_codigo") AS c_envia,
"INFORMES"."n_recibe" AS n_recibe,
(SELECT "PERSONAL"."c_nombre" FROM personal WHERE
"INFORMES"."n_recibe" = "PERSONAL"."n_codigo") AS c_recibe
FROM
Informes, personal