Not the first night:

Blood Brothers

Opened at the Albery in 1988, transferred to the Phoenix in 1991

Mrs Lyons, the programme slip states, is Mrs Johnstone tonight. The uptight middle-class business man's wife is to rough it as a put-upon Scouse single mother of god-knows-how-many and pretty convincing she is too. In reality, Lyn Paul, the usual Mrs J, is unwell and Sarah Hay, her understudy, has had to swap her regular part (as Mrs L) for Mrs J's pinny.

Pulling off such cast changes without a hitch is, of course, what sustaining a long West End run is all about (another understudy has, of course, stepped in to play Mrs L; while Mickey is not Stephen Palfreman, as billed, but Andy Snowden). If, however, a spanner has been thrown into the works of Bob Tomson's well-oiled production of Willy Russell's ambitious musical melodrama, it's to the cast's credit that the audience seems none the wiser.

As it happens, such identity swaps are just the kind of thing for which many of Russell's most memorable characters yearn. Shirley Valentine seeks redemption in the arms of a Greek waiter, a bored hairdresser thinks she'll find the new Rita care of William Wordsworth, while Blood Brothers' Mrs Johnstone believes that a twin she can't afford to bring up is better off sold to the well-to-do but childless couple for whom she cleans.

Russell's 1983 musical (now in its 10th year in the West End) charts the bond over two decades between the separated twins: Mickey Johnstone, who inherits his working-class heritage, and Eddie Lyons (ne Johnstone), who assumes a middle-class mantle. Neither realises that the other is more than a blood brother until the tragic end. Before that, Russell threads his familiar skein of themes - class, nature vs nurture, the social burden of women - through a narrative a deal more complex than his two-hander Educating Rita and a time-scale far more demanding than the one-night stand of Stags and Hens. What's more, as if attempting a Scouse mythical epic wasn't demanding enough, Russell also wrote the score and lyrics himself.

Russell's songs have stood the test of time too, the cast's crisp delivery reflecting the unfussy, loosely choreographed presence of such numbers as "Marilyn Monroe" and "Shoes on the table". With a set as simple as Russell's stage directions originally dictated, there is plenty of room for Andy Snowden and Mark Hutchinson to give full vent to Mickey and Eddie's growing pains and, in fact, it is Snowden's kinetic presence that lends the production much of its comic energy.

If Blood Brothers needs a transfusion anywhere, it's towards the end, when the fateful elements that fuel the show's broad Scouse wit - ie class and money - further complicate the romantic difficulties of an already sour love-triangle that develops between Eddie, Mickey and Linda, a childhood friend. The show seems to lose its hitherto impeccable pace and careers towards its tragic close at a bewildering rate. It's hard to know where to lay the blame: with an over-zealous cast, the writer himself or the director's desire to keep it light for the tourists and families that make up the audience. This is being a bit mean, though. After nearly 4,000 performances at the Phoenix, Blood Brothers has proved itself that rare beast - a West End musical, both poignant and witty, and well-deserving of its marathon run.

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