It depends how you look upon it. In a way both Paul and Leanne were blackmailing each other in the sense that they both threatened to tell their respective partners if one or the other blabbed. Paul and Leanne did a deal in the hotel room and agreed not to say anything. However Liam was getting more and more serious about Leanne which led to Paul becoming more and more wound up because he didn't want Liam getting hurt, again here we have the "Protection" factor with Paul another factor contributing towards his "Unhinging". This is why Paul wanted rid of Leanne plus of course he didn't like the fact that she knew his "dirty" little secret and had a hold over him. Then of course she became matey with Carla, winding Paul up even more and then they were going to buy the restaurant together, again turning that key in Paul's head:)