Dir:

Dexter Fletcher

With: George MacKay, Peter Mullan, Jane Horrocks

Runtime: 100 minutes

HOW wrong could this one have gone? It is a musical set in Scotland, the original Prozac nation. Its director, Dexter Fletcher, is a Londoner. It has songs by The Proclaimers, a duo who have raised shouty pub anthems to an art form. And it features the singing of one Peter Mullan of this parish, a chanter who makes a hungry tummy sound like Sinatra.

But what do you know, it works. Yes, the story is as thin as a bus ticket, certain accents are wobblier than a one-legged stool, and some of the background dancing would not look out of place in the early audition stages of Britain's Got Talent. What Sunshine On Leith has got, though, is heart. A big, beating, burst-out-of-the-chest tenderness shot through with gutsy humour. Gosh, golly and darn it, as they say in Musical Land, it is enough to make a Glaswegian feel fondly towards Edinburgh. Don't worry; like indigestion, it will pass.

For all its sun-dappled nature, Fletcher's picture opens in grim circumstances. An armoured personnel carrier packed with squaddies is rumbling through a patch of Afghanistan. Davy, Ally and Ronnie (George MacKay, Kevin Guthrie, Paul Brannigan) are three Scottish soldiers a long way from home and feeling every bump in the road. With the audience clued in, Fletcher does not need to linger here long (the budget would not allow it anyway). His story is concerned with what happens once the lads come home.

And what a home it looks. What Brave did for the Highlands, Fletcher does for the capital. There is a lot of geographical jiggery pokery going on, with Glasgow substituting for Edinburgh in several scenes, but the old cobbled together place looks grand.

In this fictional universe even the taxi drivers are sunbeams. As should be clear by now, a certain suspension of disbelief (from the Crown Spire on St Giles' Cathedral should do) is in order if one is to fully appreciate the charms of Sunshine On Leith.

Davy and Ally have some catching up to do: Davy with his parents (Peter Mullan and Jane Horrocks) and his sister (Freya Mavor), and Ally with Davy's sister. They are on their way from misery to happiness today, which sounds like a cue for a song, and is.

Rather like watching a pole vaulter go through their preparations, it is never difficult to know when a leap from drama into song is in the offing. While the run-ups in Sunshine On Leith are a long way from being as smooth as those in Singin' In The Rain or the recent Les Miserables, the movie gets away with it through sheer chutzpah. Talk of Florida? That'll be Letter From America popping through the letter box soon. Word of an engagement? Strike up the band for Let's Get Married.

As decisions are made about the future, the film settles into outright soapiness, the better to shoehorn in unlikely plot twists and crowbar in more songs.

The focus until now has been the youngsters, but now the dramatic burden shifts to the oldies. Rab and Jean (Mullan and Horrocks) have a landmark anniversary coming up, and the event is set to be memorable for more than Mullan's singing. (Actually, his rendition of Oh Jean is not half bad. It's elsewhere that he lifts the Pierce Brosnan "Mamma Mia!" prize for warbling that comes close to cruel and unusual punishment.)

Horrocks's singing is of a different class, with her performance of the title song enough to bring a tear to a shark's eye. It is such a beautiful, quintessentially Scottish song about love, redemption and faith in the future, that to get it wrong would have been a sin indeed. Horrocks cradles it like a baby. All of which goes some way to making up for her accent, which staggers around in some dialectical no-man's land between Edinburgh and Oldham with occasional detours to the Borders.

It is a rare blip in a film that, despite its director being unable to vote in the independence referendum, is as Scottish as tooth decay. For that, thank the Scots writer Stephen Greenhorn. Whenever the tale looks like surrendering too much to cornball sentimentality, Greenhorn's screenplay is there with a hefty wallop of McSarcasm, a raised Caledonian eyebrow, or, in the case of one little scene stealer, a cheeky Scots child so cute he could have been fashioned by Pixar.

All credit to Fletcher, though. We knew from 2011's Wild Bill that he was a director who could shake up the po-faced, self-regarding, often misery-obsessed British film industry and deliver audience pleasing cinema, and so he proves here. He is a born showman, no bad thing when making any picture.

Salutes, too, to the young cast (Antonia Thomas and Freya Mavor especially), for throwing themselves into the singing and dancing with such unashamed glee.

Finally, kudos to the Reid twins for songs that make a fitting soundtrack to many a life, Scottish or otherwise. Scotland with guaranteed sunshine. Whack that if you can.