21. No person shall, by virtue of the last preceding regulation, be entitled to have his case considered for compensation for any loss suffered by him by reason of his not being re-employed or employed as therein mentioned unless within a period of two months after the date on which he ceased to be engaged in war service he gives notice in writing to his former employer or to the authority to whom he would have been transferred by these Regulations, as the case may be, that he is available for employment;