Marcus Farrow is an agent for MI-6.
Prior to becoming an agent, indeed prior to the first book in the series, Farrow was "in advertising" in that he worked for a company who monitored the advertising their clients did. He would travel around his region, mainly Manchester and Leeds, clipping ads from various newspapers and compiling reports. It was rather mundane and unexciting work for a man who had once been a Royal Navy Commando but it paid the bills. His personal lifestyle was simple; he lived in a caravan (small trailer to us Americans) a hundred yards from his favorite pub. He has a sister with whom he is quite close, a sister with the odd name of Psyche just as his full name is Marcus Aurelius.
In the first book in the series, he witnessed a young woman being killed as a pedestrian hit by a car while fleeing a bar in Manchester. At the inquest he heard her reputation sullied by several witnesses and didn't believe it. When he was asked by a grieving father to speak up, he chose to do considerably more. It was this more that brought him to the attention of MI-6 who were into the perpetrators for different reasons.
Once he became an agent his old training comes back but he is by no means the prototypical agent and he works with others more experienced. Soon, though, he does learn what he needs to and becomes one of the key agents the bureau has.
He never loses his cockiness. He never stops liking the odd pint or the occasional whiskey. He doesn't stop needing a bit more money to live on. He stays himself, only more so.