A person who immediately before the commencement of this Act was detained under section eleven of the Lunacy Act, 1890, may continue to be detained until the expiration of the period of seven days mentioned in that section or, if at the commencement of this Act that period had expired and a petition for a reception order was pending, until the expiration of the period of twenty-eight days beginning with the commencement of this Act or until he becomes liable to be detained or subject to guardianship under this Act, whichever occurs first, and may be so detained in any place in which he might have been detained but for the repeal of that section.