Follow you home by Mark Edwards.
Synopsis from Amazon.com
The page-turning psychological thriller from the author of #1 bestsellers The Magpies and Because She Loves Me.
It was supposed to be the trip of a lifetime, a final adventure before settling down. But after a perfect start, an encounter with a young couple on a night train forces Daniel and Laura to cut their dream trip short and flee home.
Back in London, Daniel and Laura vow never to talk about what happened that night. But as they try to fit into their old lives again, they realise they are in terrible danger—and that their nightmare is just beginning…