The Riddle of the Deplorable Dandy

The Riddle of the Deplorable Dandy: A Novel of Georgian England by Patricia Veryan
Elspeth Clayton's family has been living in considerably reduced circumstances, and to improve their finances, her brother Vance becomes a soldier of fortune. His assignments take him to France, where he is caught up in political intrigues. Injured in an attempt to escape a troop of dragoons, he is arrested and held in a French gaol for questioning and probable execution.
In an effort to save her beloved brother, Elspeth turns first to his best friend and then to her most devoted suitor, but both are prevented from coming to her aid. Meanwhile, she has unintentionally antagonized Gervaise Valerian, a quick-tempered dandy, much admired in Town, but whom she finds far from enchanting. Valerian has devised a daring plan to smuggle his father, a fugitive from justice, out of England. When his accomplice in the scheme is rendered helpless, he blames Elspeth.
Despite their mutual antipathy, they are each desperate to aid their loved ones, and with considerable reluctance they eventually decide to work together. Hunted by authorities on both sides of the Channel and pursued by unknown assassins, their efforts are fraught with danger but they persist with their struggle, in the course of which their feelings for each other undergo a marked change.