Assessment of unregistered persons: tax authority may assess liable non-registrants to best judgment after notice and hearing. If a person liable to register fails to do so, the proper officer may assess that person's tax liability to the best of his judgement for the relevant periods and issue an assessment order within the statutory time limit measured from the due date for filing the annual return for the year concerned; no assessment order may be passed without serving a notice to show cause and providing a reasonable opportunity to be heard.