Our House, Cambridge Theatre, London, **<br></br>My Fair Lady, Theatre Royal, Druary Lane, London, ****

Recasting is a delicate business, particularly in long runs. There's the right casting – that can transform a show – and there's what I would call opportunistic casting. Good examples of both are currently on view in London's West End.

The producers of Our House – the Madness musical – are clearly banking on the loyalty factor in bringing in Suggs, the front man of the band, to bolster their audiences. His mere presence on stage, moving somewhat stiffly through the proceedings in the small, narrator-like role of Joe's Dad, is probably enough to bring in the Madness die-hards, or at least those less inclined to spend up to £40 for a not always very tautly strung dramatic medley of their greatest hits.

It doesn't actually matter whether he's any good (he isn't); it does matter that he represents a little slice of pop history for a series of very quirky, very individual songs. So here's a chance for the fans to pay homage. That's the producers' thinking. That's the method in their madness.

On that level, his presence is a bonus; on every other level, you rather wish he wasn't there. Tim Firth's book has enough problems of its own in Act I: the songs are very much the thing; the parallel narrative seems about as contrived as Suggs's vain efforts to pass himself off as an actor. But he has (mercifully) little to do except be there. You can tip him an invisible nod for "Baggy Trousers" whose high-school havoc is wildly evoked in a dodgem-like set piece of skidding desks and flying bodies. Or pump along to metallic rhythms of the combustible "Driving in My Car".

With or without Suggs, Our House works best in Act II, when its big-hearted social conscience – very much in the "Blood Brothers" mould – finally gets a grip and the songs are more strongly rooted. On the night I went, the much-lauded star of the show, Michael Jibson, in the dual role of Good Joe/Bad Joe, hero and anti-hero, was off. Chris Thatcher was a creditable stand-in, pretty sharp on the quick changes of personality and clothes. But he wasn't the notice-stealing Jibson.

Substitutions, it seems to me, are becoming more prevalent in West End musicals. Are today's stars more vulnerable or merely too cosseted? Eight shows a week is increasingly the exception, not the rule. The great musical theatre stars of yesteryear will tell you that maintenance is 90 per cent of a long-run show. The stamina to get up there and do it night after night and be sure of hitting a consistent level of professionalism and inspiration. Yet here we are in times where Martine McCutcheon can win an Olivier Award for hardly ever being there.

Then again, that was a bad idea to begin with. Casting a hugely demanding role like Eliza Doolittle in My Fair Lady on the basis of personality rather than ability and technique is frankly foolhardy. McCutcheon was nowhere near the role vocally. She hadn't the range, the phrasing, the musicality. Laura Michelle Kelly, the latest (and last) fair lady to grace Trevor Nunn's National Theatre production here in London, has it all in spades.

It's like the role is suddenly new again and she, like Julie Andrews almost 50 years ago, is taking possession of it. She truly inhabits Eliza. She's funny, she's touching, she's captivating. There is fire in her belly and romance in her soul.

Kelly's six numbers sound newly minted: "I Could Have Danced All Night" is a thrill for us because it's a thrill for her Eliza, no longer able to contain how she feels. And you know how she feels about Anthony Andrews' Professor Higgins: he redefines the apparent contradiction in the term "charm offensive".

The Olivier committee must reconvene, for this is the cast that should have opened the show. You have until August to catch it.