Cameron Mackintosh first heard the French concept album of Les Miserables in 1982. "I immediately made it my business," he says now, "to travel to Paris to meet composer Claude-Michel Schonberg and Alain Boublil who'd adapted the story from the Victor Hugo classic." Cameron smelt a hit.
But he's the first to admit its unparalleled and sustained success has exceeded even his wildest dreams. Next week with a special charity gala performance at Shaftesbury Avenue's Sondheim Theatre, it will celebrate 40 continuous years in the West End, the longest-running production in London with the single exception of The Mousetrap.
A company of nearly 100 performers, including guest artists from the first four decades of Les Mis - Patti LuPone, Michael Ball, Alfie Boe, Samantha Barks, Matt Lucas, Frances Ruffelle, Carrie Hope Fletcher and Bonnie Langford - will join the celebration cast in a special finale.
As it happens, and he's the first to acknowledge this, Les Mis wasn't an instant hit. Not with the critics, at least. One critic dubbed it The Glums. "I can remember trying to call the box office the morning after the first night to confirm my worst fears," says Cameron. "The line was permanently engaged.
"When I finally got through, I was told they'd never had a response like it. The theatre-going public had formed long queues at the box office, the phones were ringing off the hook and 5,000 tickets had already been sold. Truly, we'd created the people's musical."
Word of mouth, moreover, had been helped not a little by Joan Bakewell's vox pop interviews on Newsnight with first-night audience members as they left the theatre. "I shall always be grateful to her," says Cameron. It ran at the Barbican for eight weeks. When it transferred to the Palace Theatre in the West End, it was sold out."
Whatsmore, the buzz quickly reached Broadway. By the time the show opened in New York in March 1987, advance bookings totalled a whopping $12million.
Now, Cameron has put together a special company of artists currently performing at the Sondheim until November 1.
Jean Valjean is played by Killian Donnelly ("one of the greatest ever", in Cameron's estimation) alongside Bradley Jaden as Javert, Katie Hall as Fantine, the velvet-voiced Jac Yarrow as Marius, Shan Ako as Éponine, Adam Gillen as duplicitous Thénardier with the Elaine Paige of Australian musical theatre, Marina Prior, as his equally self-serving wife.
As a sung-through musical - there are no spoken words - Les Mis boasts a slew of enduring hits, the English lyrics subsequently written by the late Herbert Kretzmer, one-time drama critic of this very newspaper.
Probably the most famous are Bring Him Home, Empty Chairs At Empty Tables and I Dreamed A Dream, given added notoriety when an unknown Susan Boyle sang it on Britain's Got Talent in April 2009 and it became the most-watched YouTube video of the year with over 120 million viewings.