A Welsh National Park development plan for a National Park which neither coincides with nor falls wholly within the area of a single local planning authority shall be a joint unitary development plan.
A Welsh National Park development plan for any other National Park shall be a joint unitary development plan if it relates to a composite area unless the composite area coincides with or falls wholly within the area of a single local planning authority.
Any Welsh National Park development plan which is required to be a joint plan shall be prepared by the authorities who will be the appropriate authorities in relation to the plan.
For the purposes of this section, an authority are an appropriate authority in relation to a joint plan if—
Any proposals prepared under section 21 for the alteration or replacement of a joint plan of a kind mentioned in subsection (1) or (2) shall be joint proposals prepared by the appropriate authorities in relation to that plan, and any direction given by the Secretary of State under that section in relation to that plan shall be given jointly to those authorities.
Subsections (3) to (6) and (8) of section 23A apply in relation to any joint plan or proposals of a kind mentioned in subsection (1), (2) or (5) as they apply in relation to any joint plan or proposals of a kind mentioned in section 23A(1).