Barnaby 'Barney' Allen is an agent with MI6.
The year we first meet him is 1935 and Allen is a steward with the Jockey Club, spending his time with one of his key interests, horse racing. He was really just a junior steward at the time, barely having gotten into it and still being fairly young (37) for such a serious position. He was not entertaining doing anything else because he had a private income - "quite a generous one".
A year late, he didn't. "It was a family trust .. and it rather unexpectedly ran out of funds. Quite unforeseeable, and no fault of one's own". That is what he tells the interviewer for MI6 when Allen applied for a position there. When asked why he was coming to them, he replies "to be frank, I needed a better-paid job and one with more prospects'. Since he was fluent in German, they took him.
Allen had been 18 years old in 1916 and joined the Guards Division in time to see action in several notable battles, including the second one at the Somme. When the War came to a conclusion he decided his chances for advancement was not too good so he mobbed out and went to Oxford for three years before joining his father in the City for a handful more years - up to the point where he knew he did not enjoy it and quit to go work for the Jockey Club. Now that the need for a new source on income was nagging him and still not wanting to return to the financial world, Allen applied to MI6.
The state of the quarter century old MI6 is an interesting one, as Allen learns upon being accepted. A good number of its people was focused solely on the threat of Communism both from the Soviet Union and from Britian's own communist party while another cliche was certain the danger to the country came from the growing threat from Germany under the three-year-old reign of Hitler. Yet another group concentrated on various threats to Britain's control of the oceans. And leading MI6 was a Director who answered to the whims of the Foreign Office who above all else wanted to make sure no trouble with anyone was caused by the department's snooping.
Since any spying on German by the agents in residence in the embassy and consulates would likely be immediately discerned by the diplomatic staff and sent to the Foreign Office which would likely insist it stop, the Director has ordered the establishment of a small cadre of agents working inside Germany but independent of anyone else. Each of these operatives would work to set up his own group of assets in a network that only he would know about. This is where Allen's new role with the Firm will come in. He would be one of that small cadre.