Catesby is an agent with MI6.
William Catesby is an agent who loves his country but not the people running it. He is hated by the Americans and respected by the Russians and is not much caring about either.
Coming from a Suffolk fishing village, Catesby does not fit in well with 'Ox-bridge' crowd who run British Intelligence and he would have been forced out years before except that he is annoying good at getting the job done. Luckily (if that is the proper word), his boss, Henry Bone, know how valuable he is.
From the late beginnings of the Cold War (early 50s) when many in the halls of MI6 were really agents for the Soviets through the turbulent 60s and into the changing world stage of the 70s, Catesby ages and matures and gets grayer both literally and figuratively.