If all the works authorised by article 3 are not completed within 50 years from the coming into force of this Order or such extended time as the Secretary of State may, on application of the Commissioners, allow, then on the expiry of that period, or such extended time (as the case may be), the powers conferred by article 3 for constructing and maintaining such works shall cease to have effect except as to so much of them as is then substantially commenced.

Nothing in paragraph (1) shall affect the carrying out of works under article 5.