During the Second World War he headed the codes office supporting resistance agents in occupied Europe for the secret Special Operations Executive organisation.

From this early interest, he demonstrated his skill at codebreaking by deciphering the secret price codes that his father wrote inside the covers of books.

[1] The bookshop subsequently became famous as a result of the book 84, Charing Cross Road, which was based on correspondence between American writer Helene Hanff and the shop's chief buyer, Frank Doel.

[1] Marks was conscripted into the British Army in January 1942[2] and trained as a cryptographer; apparently he demonstrated the ability to complete one week's work in decipherment exercise in a few hours.

[1] Marks briefed many Allied agents sent into occupied Europe, including Noor Inayat Khan, the Grouse/Swallow team of four Norwegian Telemark saboteurs and his own close friend 'Tommy' Yeo-Thomas, nicknamed "the White Rabbit.

"[3] In an interview which accompanied the DVD of the film Peeping Tom, Marks quoted General Eisenhower as saying that his group's work shortened the war by three months, saving countless lives.

According to his book, Marks wrote the poem in Christmas 1943 about a girlfriend, Ruth, who had recently died in an air crash in Canada;[8] supposedly the god-daughter of the head of SOE, Sir Charles Jocelyn Hambro.

The pressure could cause agents to make mistakes encoding messages, and the practice was for the home station to tell them to recode it (usually a safe activity) and retransmit it (dangerous, and increasingly so the longer it took).

In response to this problem, Marks established, staffed and trained a group based at Grendon Underwood, Buckinghamshire to cryptanalyse garbled messages ("indecipherables") so they could be dealt with in England without forcing the agent to risk retransmitting from the field.

[3] The other side of this story was published in 1953 by Marks's German opposite number in the Netherlands, Hermann Giskes, in his book London Calling North Pole.

He argued that, despite harrowing circumstances, "not a single Dutch agent has been so overwrought that he's made a mistake in his coding...." Marks had to face Brigadier (later Sir) Colin Gubbins: Described by Tommy [Marks' closest friend] as 'a real Highland toughie, bloody brilliant, should be the next CD', he was short enough to make me feel average, with a moustache which was as clipped as his delivery and eyes which didn't mirror his soul or any other such trivia.

Marks and his wife Elena feature prominently in Hanff's 1973 book The Duchess of Bloomsbury Street, her memoir of her trip to England in 1971 in the wake of the success of 84, Charing Cross Road.

[1] Three of the poems published in the book were scrambled into the song "Dead Agents" by John Cale performed at the Institute of Contemporary Arts, London, in April 1999.