Meeting Liz Lochhead, Scotland's Makar and acclaimed dramatist, in the bar of Oran Mor on Monday morning is slightly surreal.

Time was that arriving in a Glasgow pub at such an early hour on a weekday would find you in the company of people who were, to put it diplomatically, somewhat the worse for wear. Not so at Oran Mor.

At the bar, eating his breakfast, is Dave Anderson, one of Scotland's best known actors, stalwart of the old 7:84 Theatre Company and, famously, Gregory's dad in Bill Forsyth's classic 1981 movie Gregory's Girl. Sitting nearby are doyenne of stage and screen Ann-Scott Jones and Blythe Duff of Taggart fame. And there, deep in conversation with Duff, is Lochhead herself.

The reason I find myself in such stellar company is that everyone here is involved, in one way or another, in Oran Mor's phenomenally successful lunchtime theatre A Play, A Pie And A Pint (PPP). Duff is performing in Marco Pantani – The Pirate, a play about the famous but ill-fated Italian cyclist, which ends its run today. Anderson and Scott-Jones are playing in next week's drama, Mortal Memories, which is of great significance, not only because it is written by Lochhead, but also because it will be the 300th play produced by PPP.

The play, a "bittersweet comedy" about three old people in a sheltered housing complex who find themselves embroiled in the organisation of a Burns supper, began its life as a two-character piece for Radio 4's Stanley Baxter Playhouse in 2006. After that, on the urging of Lochhead's friend, the drama director and producer Marilyn Imrie (who directs the play at Oran Mor), it became a stage piece which was given a series of rehearsed readings by Anderson and Scott-Jones at the Traverse Theatre in Edinburgh as part of the Luminate Festival of "creative ageing" in October of last year.

For all its previous incarnations, Lochhead explains, the work being staged next week at Oran Mor "is a premiere, because this play has never been seen before. The characters have been seen before. The cultural questions it broaches were maybe there in the autumn. But the play, as a drama, is happening next Monday for the first time."

Mortal Memories is, she continues, a play about "two old people [one male, one female] with very different views of what Burns had been about - They're real antagonists in many ways." Just to add to the confusion of clashing personalities, the old woman is suffering from dementia. The third character is important, Lochhead adds, "although you can't get a word out him, and he plays the piano all the time".

Fans of Lochhead's theatre work won't be surprised to learn that the piece is intended to be funny and meaningful in equal measure. "It's a tiny wee play about very big things, such as life and death. It's also a play about a good man. A good man played by David Anderson, which is a bit of stretch," she adds, with a laugh and a playful nod to the actor, who's now sitting close by reading a newspaper.

It is, Lochhead says, "a complete coincidence that I'm doing the 300th A Play, A Pie And A Pint. It's a nice coincidence." It's a serendipitous moment which has led to her to pen a verse specially for today's edition of The Herald. "It won't be a poem," she says, "but it will be a rhyming entertainment about the 300th play. I'm writing my Epistle To David [MacLennan, producer of PPP], rather than the Epistle To Davie that Burns wrote."

Lochhead was talking recently with MacLennan, another longstanding friend, about the origins of A Play, A Pie And A Pint. It all began with a chance meeting between MacLennan and Oran Mor's owner, entrepreneur Colin Beattie, on Byres Road shortly before the arts and hospitality centre opened in 2004. The combination of MacLennan's vision and energy and Beattie's commitment gave birth to a phenomenon which has lasted much longer than Lochhead, for one, ever thought possible.

"I remember him [MacLennan] telling my husband Tom and I about it [PPP] before it opened," she recollects. "As we were going up the road, I said, 'That'll be great. It'll struggle on for a couple of seasons. That's a great idea, but it won't really work out because, from Monday to Friday, you won't really be able to get enough people to turn up.' Blah blah blah. How wrong could I be?"

That was nine years ago. The rest is unlikely, fabulous history. The 299 plays staged so far at Oran Mor have been written by an extraordinary array of authors and starred such leading actors as Robbie Coltrane and David Hayman. However, no playwright is more loved at the Glasgow venue than Lochhead herself. It seems fitting that she should send A Play, A Pie And A Pint off on the next journey in its brilliant saga.

Mortal Memories plays at Oran Mor, Glasgow, Monday-Saturday, 1pm. Visit www.oran-mor.co.uk