Formless vocals haunt the overture to Blood Brothers, as the stage is set before us. Twins Mickey and Eddie walk on stage to lie down on adjacent stretchers, the end of their story. Mrs Johnstone faces away from the audience as she previews the Tell Me It’s Not True motif. The Narrator invites us in to hear the “story of the Johnstone twins, as like each other as two new pins” and suggests the blame for what has come to pass lies with a “mother so cruel there’s a stone in place of her heart”.

The quiet sadness of this opening permeates the entire show. No matter what happens, we all know how the story ends and so even moments of levity are tinged with foreboding.

This contrast between the light-hearted and the tragic is why Blood Brothers continues to sell out theatres and why I have now seen the show six times. For all the whimsy and laughter of the first act, Blood Brothers can punch you in the gut and pull at your heartstrings in a way no other show can.

Willy Russell’s Blood Brothers tells the story of twins separated at birth. One stays with Mrs Johnstone, a caring single mother struggling in poverty with seven other children. But she can’t afford to look after the second twin so gives him to the wealthy Mrs Lyons who is unable to have children of her own. The two make a pact to never tell the boys that they are a pair, on the superstitious understanding that if they find out “they shall both immediately die”.

The boys Mickey and Edward end up becoming best friends and the show explores how their differing upbringings influences their paths through life.

For the first act and much of the second act, the twins are children but still played by adults. Within seconds of Mickey (Sean Jones) running on stage, you are utterly convinced that he is a seven-year-old (ok, nearly eight). Jones plays young Mickey with ceaseless, cheeky energy. This is beautifully matched by Eddie, who Joe Sleight plays with an endearing naivety. The pair have a wonderful chemistry on stage exemplified in their performances of That Guy.

By going to university and not having to worry about money, Eddie doesn’t have to grow up as fast as Mickey. Jones gives a masterclass performance as he transitions from cheeky child to an adult struggling with depression and financial woes. At times he barely talks above a whisper, but the audience is gripped on his every word.

The Narrator (Danny Whitehead) looms over the show, appearing as a shadow to the characters at pivotal moments in their story, never quite present, never quite invisible. Whitehead brings a melancholic nuance to the role of the Narrator. He lingers in doorways and windows, watching the story unfold. At times he seems almost saddened by the inevitability of it all, which I felt was refreshing take compared to the more intense, menacing versions of the character I’ve seen before.

Of course, any production of Blood Brothers relies on the strength of the actress playing Mrs Johnstone. Niki Colwell Evans is sublime, and she plays an utterly convincing mother with powerful vocals that are flawless throughout. She delivers raw, emotionally charged renditions of Easy Terms and Tell Me It’s Not True.

This cast, more so than any other cast I have seen, excels at drawing out big laughs from the audience, without detracting from the emotional power of the later scenes.

Although the whole cast is incredible, specific mention should be given to Gemma Brodrick for her outstanding performance as Linda, Mickey’s childhood sweetheart and later wife. Her struggle to cope with Mickey’s depression is captivating.

Special mention, too, should go to Paula Tappenden’s moving depiction of Mrs Lyons spiralling battle with mental health issues.

The theme of class is the beating heart of the show. Blood Brothers asks you to consider how the class you are born into influences your whole life. I contend that Blood Brothers remains the single greatest exploration of class in all of theatre.

A second theme that results from this is about mental health. Throughout the show are multiple depictions of depression and how class intersects with this. I wonder how different the outcome of the story would be with a modern understanding of mental health and access to therapy and treatment beyond medication.

These themes, and ultimately the whole show, are timeless. The staging of this production of Blood Brothers is exactly the same as when I first saw it 15 years ago. There is no need to make changes when Andy Walmsley hit on a perfect staging first time. Similarly, Bob Tomson and Bill Kenwright’s direction appears to be unchanged.

Blood Brothers is an enduring staple of British theatre that has not, and should not, change.