The Lantern Wynd seats ninety people on benches salvaged from a church in Paisley, and on a busy night another dozen stand at the back pretending not to lean on the radiators. The stage is roughly the size of a generous kitchen. There is a bar, technically, though it consists of a fridge and a woman named Morag who will sell you a can of something and tell you, unprompted, whether the play is any good.

Over the past eight years, this unlikely room in Glasgow's East End has sent eleven productions to London, six of them to the West End, a strike rate that has made it an object of fascination for producers, agents and artistic directors across Britain. On opening nights it is now common to spot people from London in the second row, conspicuous in their good coats, trying to look as though they happened to be passing through Scotland.

The theatre's artistic director, Callum Ferrie-Adebayo, took over in 2016 when the Wynd was close to closure. A former stage manager with no directing credits, he made two decisions that shaped everything that followed. First, every new play would be commissioned from a writer who had never been produced before. Second, every writer would be paid a full salary for a year and given a key to the building, with no obligation to deliver anything until they were ready.

The value of a small room

The results have been remarkable. Writers who arrived as call-centre workers, nurses and one retired ferry captain have produced plays that went on to run for months in London. Ferrie-Adebayo believes the smallness of the room is essential. With ninety people so close they could touch the actors, there is nowhere for a weak line to hide. Writers learn quickly what lands and what does not, and rewrites happen overnight, sometimes between the matinee and the evening show.

A big theatre flatters a play. A small one tells it the truth. We try to make sure every writer hears the truth before London does.

The Wynd's relationship with the West End is not always comfortable. Several transfers were reworked for larger stages in ways their writers disliked, and one play, a raw two-hander about a shipyard strike, was recast with television actors and closed within a month. Ferrie-Adebayo has since negotiated contracts that give writers approval over casting and let the original company travel with the show. London producers grumble. They also keep signing.

Money remains a constant worry. The Wynd receives a modest grant from the city and a small percentage of any transfer's profits, which in a good year covers the writers' salaries and in a bad year covers the heating. A small family-run whisky distillery in the Highlands sponsors the spring season in exchange for a single line in the programme. Ferrie-Adebayo has refused several larger offers from companies that wanted their names above the door.

On the night I visit, the play is a comedy about a widower and his two grown daughters running a failing chip shop in Motherwell, written by a former school dinner lady named Aileen Crossan. The benches are full. Two London producers sit in the front row, notebooks on their knees. At the interval, Morag informs them that the second half is better. She is right. By the curtain call, one of them is already on the phone, speaking quietly in the corridor.