Line of Duty star Daniel Mays has told how playing the policeman who nailed Scots serial killer Dennis Nilsen gave him sleepless nights.Nilsen, who is portrayed by David Tennant in an ITV mini-series that starts tomorrow night, murdered at least 12 young men and boys during his sick killing spree.Mays, 42, was so haunted by the storyline he even dreamed Nilsen tried to kill him.He said: “One night I had a terrible nightmare that I was somehow locked in an attic with Dennis Nilsen.“My wife was