“(2) For the purposes of sub-paragraph (1) (which provides for reasonable excuse)- (b) where P relies on any other person to do anything, that is not a reasonable excuse unless P took reasonable care to avoid the failure, and (c) where P had a reasonable excuse for the failure but the excuse has ceased, P is to be treated as having continued to have the excuse if the failure is remedied without unreasonable delay after the excuse ceased.”