I would have thought unless you are singing SOE's copyrighted arrangement word for word, note for note, then the song should be public domain. Thackeray is long gone, even beyond the 90 year limit proposed by the scroungers. Cyril Tawney and Bob Roberts recorded it and McGarry is currently singing it. Neither Bob nor Cyril will consider themselves authors because they're both gone. I knew them well enough to know if they hadn't written it themselves they would never claim any rights to a song.