Michael Grant is an agent with MI-5.
One slightly confusing aspect of Grant's employment was when it is revealed, fairly early on, that he became a civil servant of the Foreign Office in 1963 at the age of 23 but was soon "detailed to the Special Branch of M.I.5". Looking further into the history of the Special Branch as I read of it growing up, I see that there have been numerous incarnations over the 100+ years it has been around, so I decided to ignore any confusion and just so with the fact that he was pulled from normal diplomatic channels and is now in the counter-spy business.
The activities of Grant take place in the mid 70's which shows that he is in his mid-30's when we start following his assignments. He comes from a pretty impressive lineage have a decorated Major General for a father and the Countess Padarewska as his mother. It was from his mother's side that he seems to have acquired his extensive knowledge of and experience with numerous parts of Europe. His acquired fluency in German, French, Polish, and Russian definitely help out in his work.
Grant's resume, which was kindly provided, shows he is into a variety of sports including boxing, fencing, and swimming. He holds a black belt in both judo and karate and he is a gold medalist in marksmanship. Also, a write-up in that document indicates that he is "willing to undertake all the toughest assignments available, a man of determination and ruthlessness yet, despite all that, one of sympathy, tact and understanding. Extremely attractive to women but avoids any 'embarrassing long associations'". [The single quotes on that last part are mine, put there because of how odd it sounds yet strangely amusing.]