“For the purposes of this Act, a person shall be deemed not to have failed to do anything required to be done within a limited time if he did it within such further time, if any, as the Board or the tribunal or officer concerned may have allowed; and where a person had a reasonable excuse for not doing anything required to be done he shall be deemed not to have failed to do it unless the excuse ceased and, after the excuse ceased, he shall be deemed not to have failed to do it if he did it without unreasonable delay after the excuse had ceased.”