Spy Another Day by Elizabeth Ellen Carter

He is a dangerous man. And his darkness runs deep…
Reduced to genteel poverty following the closure of the family’s Cornish tin mine, Alexandra Gedding and her twin are saved from the wrath of angry, unpaid workers by the timely intervention of a secretive stranger.
David Manston, the disgraced Baron of Carreg, lives as a hermit in a clifftop cottage, his true identity unknown to everyone in the village – just as he prefers. His offer of mediation was to be a one-time act of kindness, but it does not stop Allie trying to pry into his past nor lessen his own attraction to the beautiful gentlewoman.
In the midst of strange happenings at the rundown mine, a figure emerges from Allie’s childhood – her notorious godmother, Lady Abigail, who possesses the means to lay bare David Manston’s darkest secret…