Colonel Mostyn is a spymaster.
Of course, by that I mean he is the man who directs agents of one sort or another to go places and do things and come back for more instructions. He is very good at this and one of the reasons for his proficiency is that he once was one of those being so directed and he knows the kind of trouble likely to be encountered so he can better judge who he picks.
The rank of colonel is a legitimately earned one in the British Army. For more than a few years he had served Her Majesty's government in the infantry in skirmishes and out-and-out wars all over the globe. Remembering that the adventures we find him involved in take place during the early 70s and he is not a young man at this time, we can imagine that twenty years or more back was when he did his service which would put him in likely in the action of WWII and certainly during the troubles in the Middle East as those countries ended their years under the Protectorate of the Crown.
He would likely never have left his work as an infantry officer had it not been for the time "that old tank fell on my foot", a droll depiction of a combat injury that severely injured him and left with a limp he still has a couple decades later. Invalided out of normal field duty but still possessing a keen mind and otherwise very fit 6'3" lean and hard body, Mostyn moved over into the Intelligence line of work because he needed something to do and there was always the "odd job" here or there that could use his talents. He found he was rather good at it.
Mostyn could have returned to the private sector easily enough. He had come from privilege, as they say, and had a more than decent income from his family, he loved giving back and would not have seriously considered otherwise. He was described early on as one who "couldn't possibly have been anything but English". Elsewhere it is stated "Mostyn was that strange phenomenon that was thrown up only by the British Army - the poetry-reading warrior who hid a sharp professional efficiency behind a bland amateur exterior."
In appearance, Mostyn is a "long thin man with a beak of a nose" and "hard, alert eyes" which "became vague and blank at once, as though it were a special expression he wore to face the world." "There didn't seem to be a curve to his face or body that hadn't been smoothed down by years of breeding, and his hair and eyes were pale to the point of anonymity." He is prone to smoking Gaulois Riz cigarettes which he kept in a gold monogrammed case and lit "with a delicate, almost effeminate movement of his hands".
Though he might, likely purposefully, give the impression of a someone who was no danger to anyone, he could and did show the contrary at a moment's notice.