Wicked Scandal by Carole Mortimer

Lord Sebastian Trentham, Marquis of Deveril, known to his close acquaintances as Devil, is the third of the eight gentlemen known as The Sinners, tasked with discovering which of eight ladies in Society has been acting as a spy for Napoleon. He is expected to do this by fair means or foul.

Alys doesn’t know whether to be distressed or angry when her brother informs her he has invited the Marquis of Deveril, along with several other of his male acquaintances, to join him for a weeklong shoot at their manor house in Worcestershire.
The last time the marquis visited Alys’s home, she discovered him with a pistol in his hand as he stood over the dead body of her father.

Devil is not looking forward to returning to Newcomb Manor and only accepted Sir Teddy Newcomb’s invitation because of the need to clear Alys Newcomb’s name of any wrongdoing. Not that she would thank Devil for it if she knew that was his intention.
Alys believes she has every reason to dislike and distrust him, and nothing Devil says or does will convince her otherwise.

From the minute the two of them meet again, sexual tension flares into life and refuses to be extinguished. Much as Alys might fight against it. When circumstances force the two of them into a hasty betrothal that sexual attraction flares out of control.
But if Devil didn’t kill Alys’s father, then the real murderer must still be at large.