Sinclair 'Ticker' Tait is an agent with British Intelligence.
That statement, though true, has two elements that require major explanation. One is his being an operative comes about because he is coerced in the worst way to become one, not because it is something he would have ever sought on his own. The other is that the nickname 'Ticker' is not one that is actually used except to reference the stories about his adventures but this alliterative title points out the way that Tait was forced to work for Department C.
'C' is the designation for the small branch of the Intelligence community as well as the identifier for the the extremely unpleasant, utterly heartless, and totally despicable individual who runs the division. Not only does 'C' force people to work for him literally on threat of death, he obviously enjoys the hold he has on them and when one falls victim to the 'or else' end of the threat he presents to them, he gleefully inquires, "Who's next?" Not a nice man at all.
The coercion for Tait came about when he collapses while taking part in a pentathlete event in an international athletic meet. In the hospital he is told that "must have picked up a mystery virus somewhere" as it "attacked and severely damaged [his] heart". The organ was beyond repair, he was told. Then a fellow named Rogers showed up from the government and offered "a mechanical heart - made of tungsten, titanium, and other alloys which we can substitute for your diseased heart with a simple operation. There is no risk". And there would be no charge! "It is enough that we will be helping British athletics."
The fact that the new heart can be set to explode inside his chest is not mentioned until after the surgery. He is also told that he must now work for the Department as an agent or die. And that when given a mission, he would be given a very short time frame to pull it off or his heart would go boom wherever he was. That is the kind of people he is now forced to work for, or die.
Prior to his run-in with the Department, Tait, a dark curly haired handsome man, had been an enlisted man in the Royal Marines, rising to the rank of sergeant before turning down an offered commission and leaving the service to take up athletics, his great love. Tait appears to be in his late 20's, in excellent shape (other than the sabotaged heart), and possessing fantastic reflexes and instincts. Add in the fact that the artificial heart, while being boobytrapped to give him immense pain at the push of a button or explosive death should that button be held too long, was also able to pump blood and needed oxygen far better than a normal heart. This gives him added energy and resistance to some drugs and gasses, which makes Tait a terrific, albeit unwilling, agent.
Interestingly, the designation/codename for Tait is a somewhat apt 'Zombi Three', appropriate because in paranormal lingo, a zombie is a person resurrected from the dead often forced to do the bidding of the one who brought them back.