The Ninth Directive by Adam Hall

When a security exercise goes wrong, a rogue agent must defend a British diplomat from Thai assassins

Quiller is not an easy man to work with. Freethinking to the point of insubordination, he's the kind of spy who gives his superiors ulcers. But his case file, going back to his work against the Nazis, speaks for itself. The Bureau ranks him as a #9 agent-Reliable Under Torture-and that's the kind of man they need in Bangkok. Because an important British official is coming to visit, and the Bureau wants Quiller to plan the diplomat's murder.

Of course, it's only a security exercise. The official will be traveling under top-notch protection, and they want Quiller to devise an assassination plot to test the abilities of his security detail. But for the diplomat and for Quiller, the danger quickly becomes real.