It wasn’t ever going to get as far as the Whartons, though. If you imagined McGee’s murder as an Agatha Christie novel – briefly overlooking the fact that she preferred to write about the destruction of retired English colonels and genteel lady librarians rather than psychotic Loyalist paramilitaries – then the character I wanted to be was one who never appears in the book at all. He is never talked about, he is never graciously introduced to the house party in the drawing room, and so he never figures in any of M. Poirot’s magnificently complicated Belgian calculations: he can’t, because he’s not there.

That would mean, of course, that the murder is never solved. It couldn’t be, because the readers never saw the killer, or if they did, they didn’t even register his existence. It’s as if at the end of the Christie novel the murderer should turn out to be a petrol-pump attendant who was never actually mentioned, but who filled up Lady Motterly’s chauffeur’s tank during that journey to London which merited one line on page 153. It would never happen because it would make for a rotten book, and Poirot could never explain why he had alighted upon the petrol-pump attendant in the first place. That’s why it would make for such a successful crime.

The first thing I had to do was learn how to stop looking like me. The person skulking around near the scene of the murder had to appear quite different from John Mason, the cheerful, forgetful guest at a city centre hotel. I had once read an article in a Sunday supplement about how easy it was to change your appearance just by altering the shape and furniture of your face. The man pictured in the article had begun by stuffing cotton wool into his cheeks, to give himself jowls and pouches where none had previously existed. In the before and after photographs, he had turned from a trim, distinguished man to a bloke who had let himself drift to seed via the time-honoured route of ready-made TV dinners and value packs of lager.

I got up early on Friday morning and crossed the hotel lobby in a hurry, to avoid any contact with nosey Marie. In a city centre café, I ordered their biggest mug of tea and a bacon and soda-bread sandwich. The tea was powerfully strong and hot, and the springy bread melted down at each bite to hit the sinewy crunch of salty, crisp bacon. For a moment I almost forgot that something terrible had happened and another was about to happen. If only I could have frozen that wonderful moment with my mouth full of bacon sandwich and hung suspended in it like a doughy chrysalis for a month. But then I remembered McGee, and the bread jarred in my fast-drying throat and made me cough.