4:-- If in the case of any land it appears to the local authority . . . that steps should be taken for the destruction of rats or mice on the land or otherwise for keeping the land free from rats and mice, they may serve on the owner or occupier of the land a notice requiring him to take, within such reasonable period as may be specified in the notice, such reasonable steps for the purpose aforesaid as may be so specified; and where the owner of any land is not also the occupier thereof separate notices may be served under this section on the owner and on the occupier.