Season of Skulls. Book 3 of the New Management, a series set in the world of the Laundry Files

Welcome to Britain under the New Management! The Prime Minister is an eldritch god of unimaginable power. Crime is plummeting, and almost every offense is punishable by death. And everywhere you look, there are people with strange powers, some of which they can control, and some, not so much. Hyperorganised and formidable, Eve Starkey defeated her boss, the louche magical adept and billionaire Rupert de Montfort Bigge, in a supernatural duel to the death.
At least, she has reason to hope he's dead. But though she's now in charge of the Bigge Corporation, she's not free of him yet. Through the fecklessness of her brother Imp, combined with the intricate feudal law of a tiny Channel Island, it would appear that unbeknownst to her, she was married to Bigge - and that proving his death and releasing herself from his arcane bindings will take years and cost millions.
Then an emissary of the Prime Minister arrives with an offer that she absolutely can't .