Peter Senden is an agent with MI-5.
Part-time.
His day job is an ex-pat Doctor of Physics living on the Continent and far too busy to return to merry-olde England anytime soon. Situations force it, though, and with it a matter which will cause him to become involved in cloak-and-dagger work and people wanting him dead, not that he was that awfully far from it in his previous profession.
We first meet Senden when he is brought home by an urgent request stating his father was gravely ill and he was needed. The elder Senden, a Rear Admiral in the Royal Navy was not found, contrary to what Senden expected, in a rest home or hospital but in a facility on a high security military base. That let the younger Senden know things were not at all normal. Things would not be normal again for Senden for a very long time.
Senden had been, before the health summons, working for the Ministry of Defense "on a freelance basis, checking advanced engineering systems data for them". He had been, according to a person reading from a dossier on him, once working for a large defense electronics contractor before being lured by the MoD. One of his jobs for his new employer was checking on the systems used by GCHQ for communications intelligence gathering. His "vigilance" had spotted an al Qaeda bio weapons attack in the brewing and helped stop it. From this all it is easy to see that while Senden might not be trained in the cloak and dagger work, he was a pretty shrewd individual.
His type of work will change. His chance of survival doing that work will go down substantially. He will remain shrewd.