Ian Forsyth is a former agent with MI5.
He had retired from the organization "two years ago last month", as he reminds himself at the beginning of the first recorded adventure. The occassion of his recollection was yet another very friendly pull by an old friend and colleague to give up his easy life of doing not much of anything and reapplying for his old job back. This lets us know it was not his age (which is 40) that caused him to leave the job; a fact confirmed when we learn that he had decided he no longer wanted to do that sort of work when he had returned from an assignment out-of-country which had resulted in the death of a loved one.
Forsyth tells us that he had been in the military - he does not say for how long but it seemed enough to have him likely thinking of it as a career - "I was soon snapped up by the Defense Security Service once they saw what I was capable of. I had been trained by them, learned everything I knew about espionage from them, and then worked both overtly and covertly for them for years."
That friend trying to entice him back reminds Forsyth that "You loved it...The rush of the work...it's addictive". Forsyth is honest enough with himself and his friend to admit that he does miss it.
That is likely why, even though he was supposedly out of the game of investigating terrorists and spies, foreign and home-grown, he is not all that reluctant to be talked into looking into the things that will be reasons for the adventures of his that we have.
Forsyth, being of Scottish descent, has decided settle down in Kilmarnock, a ways south of Glasgow not too far from the coast of the Firth of Clyde. He chose it because while it was a decent sized city, it was far less of the bustle that he was used to while with MI5. He would find it at times not as restful as he had thought.