Logic demands it.
If Alfred Boxall was not the father of Robin, then the man who was responsible for fathering Robin was in town at the same time.
Jessica M. Harkness was born in 1921 in Essex.
Jessica M. Harkness was married to an Ernest Moulds in Yorkshire in 1943.
It is unknown if Ernest Moulds is any relation to Major William Jestyn Moulds*, but the question has been asked.
(Major Mould’s link courtesy of Gordon Cramer))
Information regarding Moulds / Harkness marriage sourced from LDS/FamilySearch site.