Interesting thing about Leo Marks, his family owned a large bookstore on London's Charring Cross Road, one which I frequently visited, probably even met him without knowing it. There is a movie about one of his most famous "trainees", Violete Zabo, who was featured in the movie "Carve her name with pride". There is a book about the "technology" of the time, available from the RSGB (and likely from CQ magazine over here which is worth a read. As soon as I can remember what it's called I'll post the title.
As an aside, Leo Marks narrowly missed getting sent to Bletchley Park and wound up in London helping with coding and teaching coding to operatives who were to be inserted into enemy territory - extremely brave folk.
Blessings all de Wayne - NQ0RP