“(1) Liability to a penalty under paragraph 39…does not arise if that person satisfies [HMRC] or (on appeal) the First-tier tribunal that there is a reasonable excuse for the failure… “(2)…(c) where the person had a reasonable excuse for the failure….but that excuse has ceased, the person is to be treated as having continued to have the excuse if the failure is remedied …without unreasonable delay after the excuse ceased.”