Greek Fire — Winston Graham

When Gene Vanbrugh discovers that a proposed business associate has been murdered he sets out to uncover the truth. His path crosses with that of Anya Stonaris, the beautiful but aloof mistress to a politician with a decidedly murky past, and their meeting acts as a catalyst to a dangerous political adventure. As the relationship between Gene and Anya grows, so does the realization that there is much more to lose than either thought possible.

'. . . never strikes a false note' Daily Mail