As the doors to the basement theatre open just after midday, the convivial queue that has expanded politely across the ground floor bar of Glasgow’s Òran Mór venue surges forward with intent. It is apparent that regular visitors have their strategies well synchronised, efficiently bagging seats with a good view of the stage while pals head for the bar area at the back to collect pints, pies and sauce sachets.

“A Play, a Pie and a Pint is just a fantastic concept,” said Pat Reid, 66, settling in for another show at one of Scotland’s most roaringly successful theatrical institutions, which offers audience members a drink, lunch and an hour of original drama that changes on a weekly basis, all for £12.50.

Reid has made the journey from Paisley, a town to the west of Glasgow, for the past four years with her husband. The pair have been regular theatregoers since they were students, but find the lunchtime slot convenient now they are retired.

“It’s great value for money,” said David Reid, 67. “You get your lunch, you get to see new plays, some you like, some you don’t, but that doesn’t matter because we’ve been coming so regularly we’re here for the enjoyment of the whole event.”

Audience members chat before the play. Photograph: Murdo MacLeod/The Guardian

This season, A Play, a Pie and a Pint is celebrating its landmark 500th play, in its 15th year. Producing 35 new shows annually, as well as summer and winter pantomimes, it has become the most prolific producer of new drama in the UK, championing emerging writers as well as tempting more established talents to try a different form.

The award-winning playwright David Ireland made his debut here in 2009, while celebrated novelists such as Val McDermid and Denise Mina wrote their first plays here. Actors including Robbie Coltrane, Karen Dunbar and Outlander’s Grant O’Rourke have starred along the way.

Founded by the great theatrical innovator David MacLennan in 2004, the idea was greeted with scepticism to begin with, said David Greig, now artistic director of Edinburgh’s Lyceum, who worked on an early season with MacLennan.

“When it started, everybody thought ‘how’s that going to work?’ and it seemed like a crazy venture. But David MacLennan identified the demand from the audience for new work. You turn up and sometimes get something brilliant, sometimes you hate it, but you love the pie and the pint and the mates you go along with,” he said.

Collecting pies in the bar area. Photograph: Murdo MacLeod/The Guardian

Without A Play, a Pie and a Pint, Scottish theatre would have been “enormously impoverished” over the past decade and more, said Greig. “Think of the new writers and the new ideas that proved themselves there. It’s also a lifeline for actors at a time when production numbers are falling,” he said.

The current co-artistic directors, April Chamberlain and Morag Fullarton, brought a wealth of mainstream theatre and television experience to the role when they took over in 2016, and have been rewarded with a doubling of the average daily audience to 170, with £10 Wednesdays and Saturdays regularly selling out. (For comparison, London’s Donmar Warehouse holds 250 people at capacity).

According to Fullarton, audiences are attracted by the location itself – Òran Mór is an imposing former church, now converted into a popular bar, restaurant and music venue – and the informality of the event.

“People feel they can come on their own, and be relaxed. A lot are first-time theatregoers, and there’s a social element too,” she said.

April Chamberlain and Morag Fullerton at Òran Mór theatre space. Photograph: Murdo MacLeod/The Guardian

The plays cover historical and contemporary subjects, Fullarton explained – a recent offering was based on last year’s art school fire – and span musicals, comedy and straight drama.

Each play has a six-day run, Monday to Saturday, and as a rule of thumb, lasts about 45 minutes and has a cast of no more than three actors. “The very nature of producing new writing is that you are taking a risk so the challenge is to keep the quality up,” Fullarton said.

Noting that A Play, a Pie and a Pint’s success has been for the most part unsubsidised – it has received Creative Scotland funding only in the past five years – Chamberlain talked about an effort of “small resources, huge imagination”, describing the relentless industry of producing a new stage design, props and costumes on a weekly basis.

The Origins of Ivor Punch by Colin MacIntyre. Photograph: Murdo MacLeod/The Guardian

“Because we can’t have a revolving stage or explosions, it’s all about the script and the performances,” Chamberlain said.

Across the aisle from the Reids, Laura McKay, 27, is taking some shots of the stage on her phone. This week’s play is another debut by Colin MacIntyre, the musician behind Mull Historical Society.

McKay works as a barmaid and waitress in Dunoon on the west coast, but is visiting Glasgow for a week and decided to come “because one of my customers raved about it”.

She added: “It’s a lot busier than I expected. I’ve not been to a play for years but it’s only for an hour and it’s something a bit different.”