It’s telling that Harry Hill’s eccentric take on ITV’s X Factor required the invention of only a handful of new characters.

And it is credit to his extraordinary powers of visualisation that, without compromising the raggle-taggle integrity of the show, or its best-loved faces, he has used the format to take us into new and unimagined realms of madness.

All the usual suspects appear in one form or another including Wagner and Jedward. As Cowell, Eastenders veteran, Nigel Harman, gets the distance and the swagger spot on, and Simon Bailey’s ironic deconstruction of Dermot O’Leary’s mannerisms, are insightful as well as funny. The ensemble is superb and a testament to imaginative casting. The only jarring note is Victoria Elliott who, as Jordy, looks and sounds too coarse to be Cheryl Cole’s doppelganger,

Es Devlin’s set is spectacular and it is easy to see why previews were beset with technical problems. Banks of individual, life-sized, cut-outs of people are used for crowd scenes. There is a towering pair of lips in which a human fly hovers, and an arum lily staircase with an extending stamen that grows to full stage height. The use of CGI is limited but forceful with beautiful montages, and there is a truly breathtaking, all-too-short, moment of wonder at the end when Simon’s true purpose on earth is revealed.

I Can’t Sing is, of course, a musical, and it is true to the X Factor ethic that Steve Brown’s score starts with a showstopper and then sticks to safe and anodyne until the final stages of the show when we get everything from rap to Riverdance. As the heroine, Chenice, Cynthia Erivo is charming, belting out diva-tunes that had the house cheering.

Director, Sean Foley, has taken Hill’s book and created a slick machine that expertly mimics the show. If you break it down into sections it’s contrived and silly and patronizing, but if you sit through it all, there’s a moment where it all makes sense in the way an out-of-body experience makes sense when you’re recalling it at a dinner party.

Are there bad bits? Certainly the first half hour is far too fawning in respect of Cowell, but it soon passes. I Can’t Sing is a must for X Factor fans, and even, possibly, for the naysayers.

****