According to her testimony, Andrea did not scream. She was not sick. She quietly backed out of the house and called the police on her mobile phone. She did not go in again until they arrived.

To begin with, the police assumed that Diana Cowper had been the victim of a burglary. Certain items, including jewellery and a laptop computer, had been taken from the house. Many of the rooms had been searched, the contents scattered. However, there had been no break-in. Mrs Cowper had clearly opened the door to her attacker, although it was unclear if she had known the person or not. She had been surprised and strangled from behind. She had barely put up a fight. There were no fingerprints, no DNA, no clues of any sort, suggesting that the perpetrator must have planned this with a great deal of care. He had distracted her and plucked the red cord off the hook beside the velvet curtain in the living room. He had crept up behind her, slipped it over her head and pulled. It would have taken only a minute or so for her to die.

But then the police found out about her visit to Cornwallis and Sons and realised that they had a real puzzle on their hands. Think about it. Nobody arranges their own funeral and then gets killed on the same day. This was no coincidence. The two events had to be connected. Had she somehow known she was going to die? Had someone seen her going in or coming out of the funeral parlour and been prompted, for some reason, to take action? Who actually knew she had been there?

It was definitely a mystery and one that required a specialist approach. At the same time, it had absolutely nothing to do with me.

That was about to change.

Two

Hawthorne

It’s easy for me to remember the evening that Diana Cowper was killed. I was celebrating with my wife: dinner at Moro in Exmouth Market and quite a lot to drink. That afternoon I had pressed the Send button on my computer, emailing my new novel to the publishers, putting eight months’ work behind me.

The House of Silk was a Sherlock Holmes sequel that I had never expected to write. I had been approached, quite out of the blue, by the Conan Doyle estate, who had decided, for the first time, to lend their name and their authority to a new adventure. I leapt at the opportunity. I had first read the Sherlock Holmes stories when I was seventeen and they had stayed with me all my life. It wasn’t just the character I loved, although Holmes is unquestionably the father of all modern detectives. Nor was it the mysteries, as memorable as they are. Mainly I was drawn to the world that Holmes and Watson inhabited: the Thames, the growlers rattling over the cobblestones, the gas lamps, the swirling London fog. It was as if I’d been invited to move into 221b Baker Street and become a quiet witness to the greatest friendship in literature. How could I refuse?

It struck me from the very start that my job was to be invisible. I tried to hide myself in Doyle’s shadow, to imitate his literary tropes and mannerisms, but never, as it were, to intrude. I wrote nothing that he might not have written himself. I mention this only because it worries me to be so very prominent in these pages. But this time round I have no choice. I’m writing exactly what happened.

For once, I wasn’t working on any television. Foyle’s War, my wartime detective series, was no longer in production and there was a question mark over its return. I’d written more than twenty two-hour episodes over a sixteen-year period, almost three times longer than the war itself. I was tired. Worse still, having finally reached 15 August 1945, VJ Day, I had run out of war. I wasn’t quite sure what to do next. One of the actors had suggested ‘Foyle’s Peace’. I didn’t think it would work.

I was also between novels. At this time I was known mainly as a children’s author although I secretly hoped that The House of Silk would change that. In 2000, I’d published the first in a series of adventures about a teenaged spy called Alex Rider which had sold all over the world. I loved writing children’s books but I was worried that with every year that passed I was getting further and further away from my audience. I had just turned fifty-five. It was time to move on. As it happened, I was about to travel to the Hay-on-Wye literary festival to talk about Scorpia Rising, the tenth and supposedly last book in the series.

Perhaps the most exciting project on my desk was the first draft of a film screenplay: ‘Tintin 2’. To my amazement, I had been hired by Steven Spielberg, who was currently reading it. The film was going to be directed by Peter Jackson. It was quite hard to get my head around the fact that suddenly I was working with the two biggest directors in the world; I wasn’t sure how it had happened. I’ll admit that I was nervous. I had read the script perhaps a dozen times and was doing my best to convince myself it was moving in the right direction. Were the characters working? Were the action sequences strong enough? Jackson and Spielberg happened to be in London together in a week’s time and I was going to meet them and get their notes.

So when my mobile rang and I didn’t recognise the number, I wondered if it might be one of them – not, of course, that they would call me personally. An assistant would check it was me and then pass me across. It was about ten o’clock in the morning and I was sitting in my office on the top floor of my flat, reading The Meaning of Treason, by Rebecca West, a classic study of life in Britain after the Second World War. I was beginning to think that this might be the right direction for Foyle. Cold War. I would throw him into the world of spies, traitors, communists, atomic scientists. I closed the book and picked up my mobile.