It was shortly after our daughter moved out that my wife and I decided to rent out our daughter’s old room. I had received a letter from Revenue and Customs demanding money for unpaid taxes, so the extra income from rent was just what we needed. My wife Linda was reluctant at first. “No way,” Linda moaned angrily. “I’m not having a stranger living here, in our house. We could be inviting a mass murderer into our home for...