The List by Mick Herron

Dieter Hess, an aged spy, is dead, and John Bachelor, his MI5 handler, is in deep, deep trouble. Death has revealed that the deceased had been keeping a secret second bank account—and there's only ever one reason a spy has a secret second bank account. The question of whether he was a double agent must be resolved, and its answer may undo an entire career's worth of spy secrets.