The Neverloving Dead by Tal Frost

It all sounds so simple: help Gethin stop a serial killer. Then Gethin will leave him alone and go ‘on’ into death. And Patrick can get on with his mission: to leave limbo in the other direction. Backwards, into life…

Repressed ex-priest Patrick died centuries ago. Now he roams London, feeding on men, building energy for a second chance at life and love. Emotions cost energy, so he avoids them.
Fun-loving Welshman Gethin struggles with anger after his violent murder. The only thing he’s ever committed to, dead or alive, is stopping his killer before he kills again. But he’s failing.
Purely to be rid of him, Patrick agrees to help. But as the clock runs down and emotions get messy, will their deal cost more than either of them intend?