The flatshare by Beth O'Leary

Tiffy Moore needs a cheap flat, and fast. Leon Twomey works nights and needs cash. Their friends think they're crazy, but it's the perfect solution: Leon occupies the one-bed flat while Tiffy's at work in the day, and she has the run of the place the rest of the time. But with obsessive ex-boyfriends, demanding clients at work, wrongly-imprisoned brothers and, of course, the fact that they still haven't met yet, they're about to discover that if you want the perfect home you need to throw the rulebook out the window.