89.—If, within 8 weeks from the date of the coming into force of the notice or such longer period as may be agreed by the planning authority and the person to whom the notice is addressed, the restoration, demolition, removal, alteration, replacement, maintenance, repair or cleaning required by the notice has not been effected, the planning authority may, subject to section 252, enter the structure or land and may effect such restoration, demolition, removal, alteration, replacement, maintenance, repair or cleaning as is specified in the notice.