As the curtain falls on every performance of The Mousetrap, the world’s longest-running play, applauding audience members are famously urged not to go on to reveal the secret solution to the murder mystery.

This autumn, however, a fresh element of intrigue has been added to the plot of Agatha Christie’s enduring hit, which first opened in 1952 at Nottingham’s Theatre Royal.

The new twist was seeded back in 1997, when an eager 11-year-old schoolboy decided to stage his own production in his school hall in Windsor. And only now is the final act being played out.

“I’d suddenly got into reading Agatha Christie and I was already obsessed with the theatre, so I bought a collection of her plays and copied pages of the script,” said Alasdair Buchan, now 37. “I really wanted to do it, but I don’t think my teachers at the small choir school that’s attached to the castle’s St George’s Chapel were hugely keen.”

The show went on regardless, for one night only, with a cast of 11-year-old boys, including Buchan, who also directed.

Buchan and the cast also travelled up to the capital to watch the real professional production a few days before their own performance. After the London show, they met the stars at the stage door. Each boy got an autograph and they promised to send them the programme they had made.

History does not record how the school play went down in Windsor, but a few weeks later the headteacher received an unexpected and stern “cease and desist” letter from the lawyers of the London producers. It threatened future action over the pupils’ recent staging.

“I was called in to see the headmaster and was terribly worried,” said Buchan.

“Back then my school managed to smooth things over and, thankfully, I was not blacklisted by the producers.”

In fact, Buchan will now join the West End cast of The Mousetrap at St Martin’s Theatre in the role of Mr Paravicini, the mysterious foreign stranger.

Buchan will take to the stage for nine shows a week over six months. “Funnily enough, when I read the script through before the audition, I remembered the lines I’d once had. Also, because I’d directed it, whole passages of dialogue came back and I was amazed how much of the structure I still knew.”

In Buchan’s school production, the eight boys played all the characters. “It was a co-ed school, but we boarders were boys. So my brother played the character of Miss Casewell.”

The schoolboys had been accompanied up to see the West End show by Buchan’s mother. “My own experience of stage doors now is that there’s usually no one there, unless you have a particular celebrity in the cast. It was manic, for example, when I was in Richard III with Martin Freeman, but most of the time it’s fairly dead.

“So I imagine the actors in 1997 were rather surprised to find eight pre-teen boys holding out our messy, A4 scripts to be signed.”

Buchan later sent in the colourful programme he had made for his show. “Some jobsworth then clearly saw it and a serious legal letter went out, demanding royalties and asking what money we had made.”

By Christie’s death in 1976, The Mousetrap had made more than £3m. But she had earlier given the copyright to her nine-year-old grandson, Mathew Prichard, as a birthday present. He later set up the Colwinston Charitable Trust in 1995 to use the royalties to support arts charities, chiefly in Wales. The show is now run by Mousetrap Productions.

Buchan, who co-founded the online theatre initiative “ReadThrough” during the pandemic lockdown of 2021, recalls often being bored at school, believing he wasn’t as musical as other pupils. For him, mounting The Mousetrap was an escape. “When I think back to it, I am amazed at the enormous amount of work we all did on it,” he said.

“And it was a success, in as far as we got from the beginning to the end. There [were] certainly boys who did not know their lines and we lost the plot a little bit at the end.

“I remember standing in the wings and trying to improve the acting by shouting at my brother, ‘Cry! Cry!’ I was quite a nice brother otherwise.”

• This article was amended on 11 October 2024. It was Richard III that Martin Freeman starred in, not Richard II as an earlier version said owing to a transcription error.