A ship whilst being navigated within any closed dock, lock or other closed work in a pilotage district shall, notwithstanding anything in this Act, be deemed to be navigating in a district in which pilotage is not compulsory.

Subsection (1) above shall cease to have effect on such day as the Secretary of State may by order appoint; and different days may be appointed under this subsection for different purposes.

Without prejudice to the generality of subsection (2) above, an order under that subsection may appoint a day for subsection (1) above to cease to have effect in relation only to such pilotage districts as may be specified in the order.