Where a local authority are satisfied of the existence of a statutory nuisance, they shall serve a notice (hereafter in this Act referred to as " an abatement notice ") on the person by whose act, default or sufferance the nuisance arises or continues, or, if that person cannot be found, on the owner or occupier of the premises on which the nuisance arises, requiring him to abate the nuisance and to execute such works and take such steps as may be necessary for that purpose :
Provided that—
where the nuisance arises from any defect of a structural character, the notice shall be served on the owner of the premises;
where the person causing the nuisance cannot be found and it is clear that the nuisance does not arise or continue by the act, default or sufferance of the owner or the occupier of the premises, the local authority may themselves do forthwith what they consider necessary to abate the nuisance and to prevent a recurrence thereof.