Where an appellant, having been granted leave to appeal under section 12 fails to lodge his or her petition of appeal within a period of three months beginning with the date on which he or she received a notification under paragraph 5 or such extended period as the Registrar may allow, the Registrar may by letter notify the Lord President of the Council that the appeal has not been prosecuted, and the appeal shall thereupon stand dismissed for non-prosecution as from the date of the said letter without further order, and a copy of the said letter shall be sent by the Registrar to the appellant or his or her solicitor and to the Commissioners.