Section 21 Content of demolition order

that the premises shall be vacated within a period to be specified in the order, not being less than twenty-eight days from the date on which the order becomes operative, and

that the premises shall be demolished within six weeks after the expiration of that period, or, if the premises are not vacated before the expiration of that period, within six weeks after the date on which they are vacated, or in either case within such longer period as in the circumstances the local authority deem it reasonable to specify.