The distinction between past and present is a rather blurry thing. My Sister Agnes novels are set firmly in the present.  She is a contemporary, detective nun, working in a hostel for the homeless in South London. But when I started writing the series in the 1990s, no one had a computer, and mobile phones were weird things with huge transmitters attached – so the early Sister Agnes stories are becoming historical whether I like it or not.

But then I started writing stories set in the real, historical past.  When it was first suggested that I write a series featuring (a fictional) Agatha Christie as the detective, I was daunted by the challenge of having to get it right – not just the historical setting, but also of her own life. In fact, it’s turned out to be completely liberating, to find myself in an altogether different era, to be able to immerse myself in the 1920s, reading huge piles of old newspapers, learning all about motor cars and fashions and concerns about the Gold Standard and Germany’s economy…

More than that, writing a real person has created an extra spark of inspiration that I couldn’t have predicted. The second novel in the series, Hidden Sins, is about the circle of artists who were employed to work on camouflage during the First World War. One of them is quoted as saying, there are two approaches to camouflage – mimicry or disguise. I thought it was an apposite metaphor for a crime novel – but more than that, for a relationship where secrets are being kept.

The novel is set in Summer, 1925 and Agatha is staying on her own at a seaside hotel in Cornwall, which is where the murder happens.

Agatha is keeping a distance, leaving it to the police. She wonders what Archie, her husband, will think when the news of the murder gets to London. She imagines him worrying about her:

‘It was an odd thought. In her mind, a picture of the golf club at Sunningdale. Her husband, standing on the green in the summer evening light, pausing, mid-swing, to worry about his wife.’

Of course, anyone who knows anything of Agatha Christie’s life at this point will know something she doesn’t, which is that Archie, in the summer of 1925, was thinking of someone other than his wife – he had met Nancy Neele through the Golf Club, and she would become his second wife.  But, and this is the key thing, my Agatha, at this point, doesn’t know this. Which means the reader, because of Agatha’s very reality, gets to know something that she, the character, doesn’t know. It gives the novel a whole extra layer of story-telling.

Writing the past, it turns out, is a peculiar privilege.  Of course, only Agatha Christie can write an Agatha Christie story. But writing her as a character has been an opportunity to reflect on what it is to find the truth within fiction – as well as to pay tribute to someone who is the mistress of our craft.

Hidden Sins is available as e-book and paperback. The first is Murder Will Out. The third, The Reckoning of Sins. is out today, December 1, 2016 – all published by Endeavour Press.