The Margin — Ian Stuart

When the London papers break a story that Walkers Brewery has been selling arms to the South African army, Graham Lorimer is put in charge of a discreet internal investigation. At a public demonstration, he meets the beautiful Shelley van Rynveld, who is dedicated to ending apartheid in her native land. Later on, a Walkers employee is killed in a hit-and-run accident and his wife tells Graham that he'd been seeing another woman - Shelley van Rynveld. Coincidence? Graham doesn't think so.