'The one bit I was quite shocked at when I first saw it was me wanking off a Nessie toy.'

The mainly unflappable Aidan Moffat is considering a moment in Where You're Meant to Be (a sort-of tour diary crossed with a culture clash between the old and the new in folk music), when even he might have gone a little too far. 'I thought, "I don't know if I want anyone to see that", but when I watched it again I thought it was quite funny. We showed it at a preview thing at the end of the Commonwealth Games and someone wrote a comment somewhere saying they thought it was me making a statement about the tourist industry. When I read that I thought, "that's hilarious, let's keep that bit in". I did worry about my mum seeing it, but then she's got all the Arab Strap records.'

There's an old story that Mrs Moffat could handle the references to drugs and sex within the lyrically frank decade-long Arab Strap oeuvre, but she could happily have done without all the c-words. And here again in Where You're Meant to Be, Moffat is rousing up the locals (from Kirriemuir to Loch Ness) with his curse-laden interpretations of traditional Scottish songs.

Among the offended is Sheila Stewart, a folk music veteran who went from being raised in the travelling community around Perth to receiving an invitation from President Gerald Ford to perform at the White House (she also sang for Pope John Paul II in front of 350,000 people at Bellahouston Park in 1982). Stewart unwittingly becomes the star of the documentary (directed by Paul Fegan, who previously earned plaudits for Pouters, his short film about fierce rivalry among pigeon racers) during which she takes great exception to Moffat's interpretation of 'The Parting Song'. He first performs it to her as they drive along in her car, a scenario which Moffat admits was utterly terrifying. 'The trouble with Aidan is that he just doesn't listen', is Stewart's withering response as she delivers her verdict that the old songs aren't fit for tampering with.

'That has been a common criticism of me,' Moffat admits of her comment. 'School was very much: "he needs to pay more attention, but he's OK when he asserts himself". But to be fair, I did a lot of research into those old songs and, as I say in the film, Sheila was a bit overly protective of that one song in particular and maybe a bit too quick to judge. As it transpires in the film, she wasn't necessarily being entirely honest about her opinions on reworking songs.'

Where You're Meant to Be is set to become the first film ever shown at the Barrowlands and will be followed on the night by musical performances from Moffat and other artists seen in the film. His original plan for the documentary was to capture his adventures within the traditional ceilidh circuit where he would deliver a few songs and poems before the locals also did a turn. Interspersed with the live action would be footage of Moffat meeting colourful characters along the route, many of whom remain in the finished film. Among them is a grieving widower who finds solace in those old songs (and is naturally another harsh critic of Moffat's versions) and the rivals on the shores of Loch Ness who have dedicated their lives to spotting the fabled monster. But once Stewart got on board, it changed everything and became much bigger than a simple 'tour diary' feature.

Moffat sensed a kindship with her for their shared love of storytelling, everyday language and the vocal delivery where 'the feeling means more than the notes'. But their light-hearted jousting leads to a wonderful finale when she takes to the Barrowland stage to interrupt Moffat's band as they perform 'The Parting Song' in order to do the 'original version' – a true gift for the filmmakers and a genuine treat for the crowd. It's a moving and spirited ending to the gig as well as the documentary, a moment made ever more poignant by the fact that this was Stewart's final performance just a few months before her death at the end of 2014.

'I'd started writing some songs and made a few trips here and there for research,' recalls Moffat. 'Everyone talked about Sheila so I started listening to her work which is quite hardcore traditionalist. We got hold of her after a while and Paul went to talk to her and discuss the film. He came back and showed me some of the footage with her and I was actually surprised at how funny she was; I didn't expect her to be so relaxed and good-humoured because she had this reputation for being a somewhat formidable woman. But she was great fun and totally up for being involved. Though I suspect Sheila wanted to be involved so she could make sure people knew what she thought about what I was doing.'

Where You're Meant to Be, Barrowlands, Glasgow, Fri 19 Feb; Glasgow Film Theatre, Wed 24 Feb. Touring around Scotland through March and April.