True colours - Stephen Leather

When a set of Russian oligarchs are assassinated in London, the Russian expat community become anxious. One man appeals to the British government for protection and MI5 assigns Spider Shepherd to the case. But he soon discovers that the killings aren't political - they're personal, and the assassin is much nearer than anyone could conceive. And that's not the only score Spider must settle. Asked by a group of ex-colleagues to hunt down a former Taliban tyrant who has claimed UK asylum by assisting the British government, Spider is forced to decide between his sense of justice and his job. For as he struggles to save the life of one man, he's on a path to extinguish another.