This is the law
JAR–OPS 1.100 Admission to flight deck
(a) An operator must ensure that no person,
other than a flight crew member assigned to a
flight, is admitted to, or carried in, the flight deck
unless that person is:
(1) An operating crew member;
(2) A representative of the Authority
responsible for certification, licensing or
inspection if this is required for the performance
of his official duties; or
(3) Permitted by, and carried in
accordance with instructions contained in the
Operations Manual.
(b) The commander shall ensure that:
(1) In the interests of safety, admission to
the flight deck does not cause distraction and/or
interfere with the flight’s operation; and
(2) All persons carried on the flight deck
are made familiar with the relevant safety
procedures.
(c) The final decision regarding the admission
to the flight deck shall be the responsibility of the
commander.
In our national legislation para (c) was not adopted.
For non-crew member flight deck jum pseats the current procedure requires the AOM to be supplemented by a instructional procedure according JAR 1.100 (a)(3), which then needs to be authorized bythe national authority. In our case it was neither tried to add such a procedure nor would the national authority authorize it.