3.  A notice given to the Board under paragraph 2 of the Third Schedule to the Act shall be so given—

(a) not less than fifty-six days before the work, particulars of which are set out in the notice, is begun;

(b) by delivering it at, or sending it in a prepaid registered letter to, either the principal office of the Board or such other office of the Board as the Board have notified in writing to the person giving the notice as an office at which such a notice may be served.