“I should have left when Biggins did,” declares the actress being played by Julie Walters as rehearsals begin for the show-within-the-show of Acorn Antiques - The Musical, and you can’t help but sympathise. Both the character and Walters herself are here to recreate her legendary turn as Mrs Overall in a stage version of a spoof TV soap of the same name. The original was immortalised as part of a BBC comedy series, Victoria Wood – As Seen on TV, exactly 20 years ago, and now (as the show begins) it’s being turned into a musical that’s beginning rehearsals at the Enoch Powell Arts Centre in Sutton Coldfield.

Except that we’re not in Sutton Coldfield at all. We’re in the plush splendour of the Theatre Royal, Haymarket, and audiences are being expected to cough up the highest prices ever for a West End show to watch this all-too-knowing rubbish. (That’s a phenomenal £65 for top weekend tickets – making the West End more expensive than Broadway for the first time in history - with £47.50 for the upper circle, £37.50 for restricted views, and the cheapest being uncomfortable bench seats of £27.50 in the remote reaches of the balcony).

Of course, part of the point of Acorn Antiques is that it’s intentionally awful. At least in that regard, it wins out over director Trevor Nunn’s last original play in London, the all-too-turgidly sincere We Happy Few which revolved around the backstage life of a classical touring company and was one of the most stomach-churningly dreadful new plays of last year. Bizarrely, Nunn now re-visits similar territory with Wood’s show, this time observing the backstage life of a hopeless musical being rehearsed, then from out-front as it’s actually produced. It’s Noises Off meets The Producers. Or rather, it wishes that it were.

This spoof-within-a-spoof relentlessly mines a one-joke TV comedy sketch. The chief pleasure of the original, re-stated in five-minute bite size chunks, was its constant repetition as a series of set pieces around the wobbly sets and dropped lines that the panicked actors had to contend with.

But in the theatre, stretched out to fill close on three hours, that joke wears painfully thin, and is filled out instead with weak pastiches of musicals from Les Mis and Company to A Chorus Line. Instead of spoofing bad theatre, Wood’s show (for which the comedienne has provided the book, music and lyrics) simply becomes it. Early on, as Walters’ Bo Beaumont arrives at rehearsals with the unopened and unread script in her handbag, she declares, “I’m going to need scissors.” If only Nunn had brought some shears to rehearsals, too.

There are a few compensating pleasures in the performances. Just the sight of Walters, stooped and shuffling as the crumpled Mrs Overall, is frequently enough to raise a guffaw. Celia Imrie and Duncan Preston – also recreating their television turns as, respectively, Miss Babs and Mr Clifford – made me smile. But these were the laughs of recognition, not revelation. However, the rest of the cast - that includes Sally Ann Triplett (in the original Wood role of Miss Berta), Josie Lawrence and Neil Morrissey - has to rely on comic affectation rather than our affection.

- Mark Shenton