The Apocalypse Codex. Book 4 in The Laundry Files

Bob Howard used to fix computers for the Laundry - the branch of the British Secret Service that deals with otherworldly threats - but those days are over. He's not only been promoted to active service but actually survived missions against cultists, enemy spies and tentacled horrors from other dimensions. Willingly or not, he's on his way up in this dangerous organisation. When a televangelist with connections to 10 Downing Street seems able to work miracles, the Laundry takes an interest.
But an agency that answers to the Prime Minister can't spy on him themselves, and Bob's shadowy superiors come up with a compromise - they hire 'freelancers', with Bob in charge. British citizens who discover the occult are either forcibly recruited by the Laundry or disposed of, and Bob's never heard of freelancers before. Officially they don't exist. Anyone who's big and bad enough to remain independent is going to be hard to handle, and Bob's not too sure that the one-week 'people management' course he was sent on in Milton Keynes is going to be enough .