A local planning authority may at any time prepare proposals—
If the Secretary of State directs them to do so, the authority shall prepare, within such time as he may direct, proposals for—
An authority shall not, without the consent of the Secretary of State, prepare proposals in respect of a unitary development plan if the plan or any part of it has been approved by the Secretary of State.
. . .Sections 12 to 20 (other than subsection (1) of section 12) shall apply in relation to the making of proposals under this section and to any alteration or replacement so proposed as they apply to the preparation of a unitary development plan under section 12 and to a plan prepared under that section.
As soon as practicable after—
the local planning authority for an area in which the zone is wholly or partly situated shall review any unitary development plan for that area in the light of the provisions of the scheme or modified scheme under that Schedule and prepare proposals under this section for any consequential alterations to the plan which they consider necessary.