It's not just an Ops Manual requirement:
Originally Posted by FODCOM 21/2010
1 Introduction
1.1 Under EU-OPS, JAR-OPS 3 and the Air Navigation Order (ANO), persons carried on board an aircraft fall into only one of two categories; crew consisting of flight or cabin crew members, or passengers.
1.2 Some aircraft certificated for single-pilot operation are fitted with a second pilot's seat. For Commercial Air Transport and Public Transport operations being conducted under EU-OPS, JAR-OPS 3 or the ANO, no person may be carried on the flight deck except a crew member assigned to the flight as an operating crew member, or a passenger permitted to occupy a flight deck seat in accordance with instructions in the operator’s operations manual. Flight deck seats include 'jump seats' and empty pilot's seats whether in a separate flight deck compartment or, in smaller aircraft, at the front of the cabin.
1.3 For larger aircraft, the National Aviation Security Programme controls access to the flight deck. This FODCOM only applies where that Programme does not prohibit the carriage of passengers on the flight deck.
1.4 The purpose of this FODCOM is to alert and remind operators of the requirements for carrying persons in flight deck seats.