The idea itself — equal parts bonkers and inspiring — could almost make a musical.

Take one beloved musical, audition more than 800 would-be stars and give the winners six weeks to learn the script.

Then book them into Theatreland’s most prestigious billing: Sunday night at the London Palladium, for a one-off, never-to-be-repeated performance.

The catch? Give the 150-strong all-volunteer cast and crew just 48 hours before last night’s show time to put the entire three-hour caboodle together — from choreography and lighting to set-building and costumes.

Then, give the proceeds — hopefully more than the £20,000 the British Heart Foundation received when the same concept was applied to Sweet Charity at the Theatre Royal in 2006 — to bone marrow charity the Anthony Nolan Trust.

The cast was buoyed by an ocean of goodwill from an audience prepared to overlook the occasional corpsing or Sally Smith’s wedding veil falling over her face as she danced with her true love Bill Snibson during the title tune.

But such was the urgency, enthusiasm and talent that this production managed to inject new life into Stephen Fry and Mike Ockrent’s 1984 revision of a 71-year-old warhorse.

This was not the moment for a radical reinterpretation so, wisely, they played this tale of class misunderstanding for sweetness and laughs.

Comedy cameos abounded, not least from Andrew Nash as an educated police constable and Paul J Hunt as the tennis-ball shaped family solicitor Parchester.

Richard Maxted’s Bill rather overdid the Rodney Trotterisms and sizzled more with Celine Dion-like Natalie Tulloch’s Jaquie than with Amy Castledine’s Sally.

But he gave Leaning On A Lamppost exactly the right mix of hubris and pathos.

Bill and Sally, the Cockney urchins adrift in the land of the snooty, were given sterling support by Carola Stewart’s Duchess of Dene and Andrew McDonald’s Sir John Tremayne.

These actors may not have known each other as recently as Friday, but they looked as comfortable with each other as a veteran company.

The big numbers — The Sun Has Got His Hat On, An English Gentleman and The Lambeth Walk — were all-singing, all-dancing, romps, just as they should be.

By the happy ending, cast and audience had enjoyed themselves as much as the other.

48 hours until the show goes on

At 7.30pm on Friday the clock started ticking and the Me And My Girl challenge began.

Never having rehearsed together or seen any planned choreography, cast and crew had only 48 hours before appearing in front of an audience of 2,000.

Considering it normally takes around six weeks to rehearse a West End show, there was no room for discussing “motivation” and director John Sheerman made clear everyone had to be focused at all times.

But somehow, from the moment the clock struck on the Friday, the machine started and two days later the cast, utterly exhausted, achieved the impossible. It went down a storm.

Bo Wilson