In all Cases in which it shall be necessary under the Provisions of this Act to serve any Notice upon any other Person, it shall be sufficient to send such Notice in a registered Post Letter, directed to such Person at his then or last known Place of Residence or of Business, unless the Letter containing such Notice shall be returned from the Post Office as undelivered; and if such Person shall not have any Place of Residence or of Business within Great Britain or Ireland, or if the Place of Business or of Residence of such Person cannot with due Diligence be ascertained, then such Notice may be served upon such other Person as his Representative, or be given in such other Manner as the Commissioners shall in such Case direct or approve.