Unforgotten writer Chris Lang reveals the real-life disastrous New Year's Eve getaway that inspired series three

In series three, the suspects are four mates who had rented a holiday home nearby with their wives and kids for a long weekend.

• Who killed Hayley in Unforgotten series three? Meet the suspects
• Meet the cast of Unforgotten series three
• Stay up to date with the RadioTimes.com newsletter

Although all four men initially tell DCI Cassie Stuart (Nicola Walker) and DI Sunny Khan (Sanjeev Bhaskar) that they had a quiet night in, it soon emerges that this wasn't quite the case; ugly memories of drink, drugs, marital breakdown and mental illness are laid bare.

"The inciting incident was slightly precipitated by a weekend away I had with four mates and our wives in 1999 – which didn’t go entirely to plan," Lang explains on the set of Unforgotten in Buckinghamshire, where Walker and Bhaskar are filming their police station scenes.

"I have to say, no one was murdered. It was a volatile time for a few people and it was New Year’s Eve and it didn’t go entirely as planned. Some things were sort of aired about where people were in their lives and relationships and emotional places.

"And I’d like to say all those involved in it are still very happily married and together and everything. But it’s an extraordinary time, New Year’s Eve, the time when you think you’ve got to be celebrating and having a good time."

After an experience like that, you won't find Lang popping the champagne corks and singing Auld Lang Syne.

"I always make a point of going to bed – I sound like a really fun guy – I go to bed at exactly 20 to 12 and go, 'F**k off, the lot of ya!' So that’s my Happy New Year," he laughs.

Reassuringly for Lang's friends, the collection of characters played by Alex Jennings, James Fleet, Kevin McNally and Neil Morrissey are not actually based on real life, even if their holiday served as inspiration.

"Of course, it’s nothing to do with them at all, but it sparked off quite a good starting point for the story’," Lang explains. "I’ve extrapolated it to the nth degree and taken it to a very different place, but as an indicator of how people come together when they’re very close friends, and how that can sometimes not work, and how it can precipitate seismic events – it seemed like a good starting point."

This starting point enabled him to do something slightly different in series three.

"I wanted to explore friendship," he says. "The four characters who find themselves under the spotlight in this show are four people who’ve known each other since they were school friends.

"I have a similar dynamic in my life. I have four or five of my best friends I’ve known since I was 11. And I’m 56 now, so I’ve known them for 45 years and we’ve been through a lot together and our partners and our wives have all been through all of life’s vicissitudes over 45 years – nearly half a century.

"In previous series, the suspects have appeared to be unknown to one another and sometimes are unknown to one another, but this series I wanted to explore what it was like when four people who knew each other incredibly well – and who thought they knew everything about their mates, and had been through all the things that anyone who has had a friendship that long has been through – when they begin to suspect that that’s not the case and there are other things they have hidden beneath the surface."

He adds: "It really was digging deeper into central theme of Unforgotten, which is how well can you truly know someone."

Unforgotten continues on Sundays at 9pm on ITV

Sign up for the free RadioTimes.com newsletter