Thomas Dylan is an agent with the British DIS.
That acronym stands for the Defense Intelligence Staff which is fairly analogous to the American DIA in that it is an Intelligence gathering organization that is an integral part of the military. The DIS is comprised of both civilians and military personnel and, according to one description I read, its primary role is being an "all-source analysis", which means it deals in both HUMINT and SIGINT. Unfortunately for the people who work at the DIS and who also have to have contact with those from MI5 or MI6 or GCHQ, it quickly become apparent that the DIS is considered and treated like the proverbial red-headed step-child.
[It is useful to point out here that these adventures take place in the 1970's. It is common to refer to periods a half century ago as being "simpler times". They definitely were not as any perusal of newspapers or magazines will clearly show. The world was going to hell in a hand-basket back then every bit as much as it is today, it is just without the Internet or cellphones.]
Dylan attended Durham University in northen England and his tutor (advisor?) there thought he might make a useful candidate for MI6 but after an interview with a visiting headhunter, he discovered they had passed on him but had the decency to pass on his resume to the DIS. That group was interested because while he was in school he had joined the Officers' Training Corps and so while he had not opted for a military career, he had certainly shown a leaning in that direction.
The final pep-talk he got in the interview was interesting: "The job is desk-bound and the pay is miserable but it's important and it's yours if you want it."
The interesting part comes from the fact that the statement came from the DIS Director and he meant the part about it being desk-bound but people who ride a chair don't often get people trying to kill them and they certainly do not get people trying to kill them often. Dylan does and it is not just because his witty observations can be annoying to many people (but never to me - I love them).
Dylan does not actively seek out danger. He is neither unduly bold nor is he particularly foolhardy. He does his job like he is told to but he does have a pretty good head on his shoulders and a decent curiosity which lend a hand in getting him into trouble which in turn leads to those people wanting to do away with him.
Playing a very important part in the adventures as well as in Dylan's personal life is Julia. She is decidedly her own person and she has a brain and a temerity that is impressive to watch. And her skills will prove very useful to Dylan to not only accomplish his missions but to also stay alive. Useful, that.