Scottish forensic expert Enzo Macleod returns in Peter May's sixth and final addition to the Enzo Files. In the first book in the series, Extraordinary People, published back in 2006, Enzo makes a bet that he can use forensic science to solve seven of France's most notorious unsolved murders, as detailed in journalist Roger Raffin's book Les Assassins Caches (The Hidden Killers).

Eleven years later May is pleased to have finished the final installment, Cast Iron. 'It feels good, it was a long time coming. I actually had a period set aside in my schedule to write the book five years ago and I got into a legal dispute with my then US publisher which was a stressful and time consuming thing and that window of opportunity to write the book closed on me. I didn't know when I was going to get it written and there were a lot of people out there who had read the first five books.'

Fans of the series will be rewarded for their wait, with the sixth and seventh unsolved crimes tackled in Cast Iron: twenty year old Lucie Martin whose remains were discovered in a lake and the murder of Roger Raffin's own wife. A final case that's especially sensitive as Raffin has a young baby with Enzo's daughter Kirsty. However the novel isn't just for those who have followed Enzo from the beginning. 'A lot of people weren't going to have read the previous books so I was very conscious of having to make it work as standalone. It was quite difficult as there are a lot of threads running through the series: attempts on his life, relationships, that I had to weave into Cast Iron so that people understood where it was all coming from and how it was going to resolve itself.'

He describes the forensic expert and wine lover as one of his favourite characters, as the books are lighter and more humorous than some of his other work. 'I was a bit nervous about coming back to Enzo after the break because I wasn't sure if I would be able to write for him as I had before. I reread all the previous books immediately before I wrote this one. That's not something you would normally do. You forget a lot of the things but Enzo just jumped out at me. He's got older and wiser because of the events that have taken place during the series, particularly in relation to his two daughters and his rather complex love life.'

Even though May knew from the beginning that he was going to set Enzo against the seven cold cases he didn't have a fixed idea as to how the series would pan out 'I had a list of ideas for each of the individual books but they were just one line ideas. I didn't know when I set out to write the series who would turn out to be the baddie in the end. I didn't come to that realisation until about halfway through the series, the way that character and relationship had developed I thought, "oh it has to be them". Having had that realisation I started working on what I'd already written and developing that thread in a kind of subliminal way in the other books so that the ground work was done for that revelation in the final one.'

Researching the novels has taken May all across his adopted country, 'one of the reasons I started writing the series in the first place was as an excuse for me to go and explore France', and required a lot of specialist knowledge from experts, 'I've been amazed how generous people are with their time and their information over the last 20 years.' When May needed information about provincial Gendarmerie he just went up to a station, which was surrounded by all sorts of security and rang the bell. 'A voice came over and asked what I wanted and I said, "I'm a writer and I'd like to talk to a Gendarme about solving crimes", you could almost feel the question mark appearing above the person's head. I walked into this reception area and there were a couple of Gendarme looking at me as if I had three heads. They obviously had never had anybody come asking this kind of thing before, not least a daft foreigner. A Gendarme took me into his office and he turned out to be the most extraordinary character who I based the Gendarme in the book on, he was just a gift.

Cast Iron is published by Quercus on Thu 12 Jan.

Peter May discusses his book at Jarrolds, Norwich, Thu 12 Jan; Waterstones, Perth, Mon 16 Jan; Edinburgh Central Library, Edinburgh, Tue 17 Jan; Partick Burgh Halls, Glasgow, Wed 18 Jan; Waterstones, Manchester, Thu 19 Jun; Waterstones, Oxford, Mon 23 Jan; Waterstones, Kensington, Tue 24 Jan.