You wouldn’t know by looking at it, but Stephen Sondheim’s Follies would not have been the show it is, and may never have been a show at all, were it not for Fiddler on the Roof.

The story goes that Sondheim and book writer James Goldman had put together the ingredients for a musical thriller in which the main protagonists, two married couples, each had a reason to commit murder. The occasion was always going to be a Ziegfeld Follies-type reunion in the setting of a disused theatre, but it was only when Sondheim and Goldman attended the first anniversary party of Fiddler on the Roof, which was held on the stage of New York’s Imperial Theatre, that the penny dropped. They had over-plotted.

Sondheim had suggested that he and Goldman sit in the stalls and watch the party from a distance. One man took a bite of a sandwich, grimaced at the taste but couldn’t find anywhere to deposit the rest. So he dropped it into the orchestra pit. That was the moment that Sondheim realised the show should be more about the human condition than a whodunnit — or who will.

The focus is still on two couples. Sally (Imelda Staunton) and Phyllis (Janie Dee) are former show girls married to the men who used to court them backstage; Phyllis’s husband Ben (Philip Quast), and sally’s Buddy (Peter Forbes). Goldman retains the conflicts of his original idea: Sally always loved Ben and still does, while Ben used to love Phyllis but now wants a divorce. As the party progresses what is expressed here is not so much murderous intent as fathom-deep regret. And no one is more trapped in that condition in Dominic Cooke’s utterly exquisite production than Imelda Staunton’s Sally.

This is the third consecutive stage performance in which Staunton, an actor about whom it could be said exudes a quintessential Englishness, has stormed some of the most challenging of American roles in the canon. Her Mama Rose in Gypsy was followed by her Martha in Who’s Afraid of Virginia Woolf? Both performances have gone down as two of the best those roles have ever manifested. And now, here, as the lovelorn Sally, she makes it a hat-trick, capturing the hope and humiliation of unrequited love, conditions that combine to evoke a devastating sense of loss when she sings Losing My Mind.

But it’s not all a downer. This is a show whose life-blood is the spirit of Broadway. And no song captures that sentiment better than Tracie Bennett’s Audrey-Hepburn-like Carlotta when she sings the endlessly witty I’m Still Here. Well, perhaps Janie Dee’s stunning turn in a dance set piece that belies Phyllis’s — and maybe Dee’s — age. Oh, there’s also the moment in which the former chorus girls reprise one of the numbers, are then joined on stage by their younger selves, and in a moment of ravishing poignancy link arms with the alter and older egos. Pure magic.