I love the fact that your first book is Endless Night. It’s not that well-known, but it’s a real page-turner.

The book is about three young people; my grandmother was well into her 70s when she wrote it. As a young person myself in the early 1960s, I saw more of my grandmother than at any other time because I was studying at Oxford, not far from where she lived and worked at the time. I used to take my friends to see her, often for Sunday lunch, and I think I even took my first girlfriend. She didn’t seem like an old person at all. She was always interested in what we were doing and fascinated by our relationships with each other. She never judged us.

There are, fortunately, no characters in Endless Night that she ‘copied’. But she does say somewhere in the book that it’s all about relationships. To me, it’s an astonishingly modern and human book. To write about people 50 years younger than herself was a tour de force. And that’s quite apart from the plot. It also illustrates her belief in evil as a force in the world.

The plot is indeed extraordinary.

Yes, though you could say that about many of her books. I would have chosen more than five if you wanted all the ones with wonderful plots.

It’s true. I’ve read a lot of mysteries and thrillers in my time and, plot-wise, I really don’t think anyone has ever managed to better her. People do try, but…

It’s one of the reasons her books have stood the test of time. My wife died five or six years ago and I’m now married to someone much younger, with nephews in their early teens. They love Agatha Christie, and it’s not just because I’ve entered their lives vicariously.

Your next choice is The ABC Murders. You’ve chosen the unabridged audiobook, read by Hugh Fraser. Why’s it on your list?

I thought it would be nice to have an audiobook. Agatha Christie is one of the very few people to have sold more unabridged audiobooks than abridged ones. That says something about the quality of her writing and the difficulty of abridging detective stories. You can’t leave out the red herrings. Hugh Fraser reads it wonderfully. In many ways, it’s the most complicated challenge Poirot faced. When you come to the end, you really do put down the book and say to yourself: ‘Why on earth didn’t I think of that?’

There was always a copy of the ABC railway guide beside my grandmother’s telephone. I’m sure that’s where the story originated. I can see her sitting there, talking to one of her friends and staring at the book, the plot forming in her mind. It’s always been one of my favourite Poirots. It had a very good TV representation by David Suchet.

Is it true that she got rather fed up with Poirot?

Yes. It’s common knowledge she wrote her last Poirot in the 1940s, in which he comes to a sticky end. Her agent and publisher persuaded her to put it in a drawer and it wasn’t published for 30 years. Part of Poirot’s attraction is that he is irritating and pedantic. He likes his boiled eggs the same size and he’s dandily dressed. If you’re writing about someone like that, he’s probably going to get to you. The other reason she found him tiresome was nothing to do with him at all. Her mind was so fertile. She was constantly thinking of plots and stories, most of which weren’t suitable for Poirot. She was dying to get those written down. She would ring up her agent or publisher in the 1940s, the era in which the Poirot books were at their most popular, and one or the other would say: ‘It’s wonderful, Mrs Christie, but couldn’t you write another Poirot instead?’ I think that annoyed her too.

She wrote many wonderful books without Poirot or Miss Marple in them.