Written in-house and first performed in front of the People's Princess alongside a star-studded bash that shut Euston down, Chickenshed Theatre's reproduction of the musical that marked their ascendancy to officialdom twenty years ago is currently being run until January 11 to mark the company's 40th anniversary. This modernised remake of their tall-tale mingling Peter Pan thematics, Scrooge, Pantoland and an Ice Queen is a glorious melee comprising 800 sassy ingénues – of all walks, stripes, colours, costumes and abilities – and reminds you why we celebrate Christmas at all.

Nearly 40 years ago in 1974, founders Mary Ward and Jo Collins approached Lady Elizabeth Byng, chatelaine of Wrotham Park – made famous in the filming of Gosford Park – with a grand vision, laid out very humbly. Seeking a rehearsal space, their idea was to help troubled youth – people not usually associated with the dramatic arts, the marginalized in any form – a chance to act and volunteer and form a community and hopefully redirect their lives. Lady Byng's only available building was her chickenshed. There were still feathers and damp and unholy smells in the air when they first started rehearsals. 

By the late eighties the initial risk of that partnership had been fashioned into a thriving community in need of greater patronage than Lady Byng could offer. The troupe needed a building of their own. The Royal Patron, Princess Diana, mentioned it to her friend, who was the wife of celebrated property developer and philanthropist, Baron Max Rayne, who would eventually become the patron and the namesake of the Chickenshed's new HQ, Rayne Theatre.

At the launch of their new building in 1994, The Night Before Christmas was the inaugural production that broke the bottle over the building's bow, so to speak. 20 years after that first performance and nearly 40 years after they occupied an aristocrat's chickenshed, the collective has fanned out to include 20 accompanying "sheds" throughout the UK and another two more in Russia. There is a considerable waiting list for entry. Students without qualifications can get on and get trained for the creative workplaces, most of whom would not otherwise qualify for institutional requirements.

Yet 95% of these youths graduate with a Btec and make useful, happy lives. Some 70 full-time staff are now required and the organization keeps growing, as do its celebrity patrons which include Dame Judi Dench, Cliff Richards and Kenneth Branagh. The shed now turns over 3.6 million a year and is self-sustaining. What astounds me is this: most of the current staff either studied themselves, or had their children go through the training. 

The play is pretty good too. It tells the story of a Faustian pact with a happy ending and features everything from sass-talking ten-year-olds, acrobats suspended off castle walls, an Ice Queen literally rolling onto the stage inside a crystal ball and two Ugly Sisters (men in drag, reminiscent of Sacha Baron Cohen's performance in Les Mis). Oh, and children on magic sleighs (read sofa) who boss reindeers (grown men in onesies) around, clutching i-Pads and delivering punchy one-liners: "Everybody knows Father Christmas is just a marketing ploy invented by Coca-Cola," says a tiny girl to her governess. 

Six actors of varying abilities and backgrounds – the Sleigh kids – make up a family which travels to distant and magical lands to return presents which Santa has mistakenly allocated. Spoiled as the children are, the audience follows them and even sometimes joins in by way of sing-alongs with the Ugly Sisters, reminiscent of an Eddie Izzard or Dame Edna routine but much softened to reflect the audience. For my money, the most visceral moment of the play was in some sense the simplest and the oldest and the most humanising: watching a delightful young actor with cerebral palsy capturing Bob Cratchit in all his generosity and wounded humanity.

My only problem was with the updated script's inclusion of modern technology, which the kids are alll using, and there are some slight kinks on the narrative arc that the writers might want to iron out. But leaving aside these minor quibbles, the theatre's unique features – such as a hand-sign mime and the grand costumes and live band – provide ample charge to keep the crowd's interest. A rare Christmas treat. 

The Night Before Christmas at Chickenshed, at Chickenshed

Twenty some years after Princess Diana helped a group of young actors save The Night Before Christmas, Chickenshed Theatre's inclusive values hold true. A Christmas show that signs the way to a more compassionate society – with lots of exciting theatre along the way.