The Flat Share by Beth O'Leary

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, wrongly imprisoned brothers and, of course, the fact that they still haven't met yet, it seems that flatshare is more complicated than expected...