Roger Tallis is an agent for British Intelligence.
The exact title for the department in which Tallis works is the "Directorate of Special Services", but it is known better in the Intelligence community as Charlie's Bar. The man known as Charlie controls a relatively small but highly effective group of operatives who, like Tallis, saw action elsewhere but who were drawn to the charismatic leader because he got the job done. He was also known as a leader who looked after his people.
Tallis, in one book, mused that the concept of "out there you are on your own" never applied to those who worked for Charlie because his influence and his connections were on call when needed. Be it help from MI-5, MI-6, Special Branch, or a foreign service like the FBI or CIA, Charlie could make a call and get the agent the help that was needed. With that in mind, though the agent also knew he would have to explain afterward to Charlie why he needed help, not something any of the agents wanted to do.
Tallis is in his late 30's or early 40's at the start of the series and has worked for Charlie for many years already. They have a good relationship built on mutual respect and from having served together in combat.
Though he works for an Intelligence organization, Tallis does not consider himself a spy. He is not one to try and collect secrets from the other side nor is he particularly interested in protecting secrets from them. What the men and women who work at Charlie's Bar do is to stave off embarrassment. Whenever activity which could be considered harmful or embarrassing to the British government, no matter where in the world this activity might occur, it is up to people like Tallis to sort it out as best as possible. Sometimes that action is criminal in nature and sometimes it is not. Sometimes it is clandestine operations and sometimes it is quite open. Whatever the cause, though, those who work for him know they would not be sent if it wasn't important.