Harry Tate is an agent for MI-5.
He has been an agent in fairly good standing for quite a few years, certainly more than a decade as he is in his early forties as the first book begins. The comment about his standing is accurate as his peers and immediate supervisors believe him to be a very capable and trustworthy agent. The same high praise is not so forthcoming from to the higher-ups who have dealt on occasion with Tate's honest answers to questions for which honesty is not sought.
Prior to his years with the counterintelligence organization, Tate had served in the British military, reaching the rank of Captain through decorated service in several hot zones such as Kosovo and Iraq. He was regarded by his superiors there much as those close to him in MI-5 saw him - diligent, capable, intelligent, and usually counted on to do what he was told "most of the time". Now in the department, he found he liked what he did and had no great desire to move up the food chain and out of where he was best qualified. Such apparent lack of ambition, though, made him suspect to some above him.
Tate probably had no special changes to his life planned as the series opens but changes are soon coming drastically when a rushed assignment without sufficient manpower goes terribly wrong when two innocents sneak into an area they were not permitted to go and then had the indecency to get themselves quite dead. With the press poised to raise a huge stink, a fall-guy was needed and Tate's truthfulness would prove too much of a burden.
It might have been better for all concerned if they had just sent Tate on a long paid vacation instead of what they chose to do. Tate can be easy-going up to a point but crossing that line is a dangerous act as he would soon show them.
As the series progresses, Tate finds that though he is not in the employ of MI-5, he cannot escape his training and his skills. The person signing the paycheck may change over time but the work and dangers and the excitement stay the same. Another thing that does not change is Tate's abilities as a finder - someone who can track down other people who might not want to be found. He is very good at it.