This, sadly, is not a biopic of the Proclaimers, but a family tearjerker adapted from Charlie and Me, Mark Lowery’s novel for older children, an adventure about a teenage boy who runs away from home with his little brother to go to their grandad’s. It’s a sentimental film that requires a cast of fine actors to squelch through some fairly heavy slush. Among them, Bill Nighy as the grandad seems to suffer from some kind of reverse Samson effect with a rugged beard that might be to blame for his charisma dip.

The film switches between time periods. In the present, teenager Finn (Roman Griffin Davis) runs away after overhearing his separating parents (Clare Dunne and Michael Socha) arguing about who gets which child in the split. Finn takes his scampish younger brother Charlie (Dexter Sol Ansell), and off they set on a 500-mile trip from Sheffield to Dingle on the west coast of Ireland where their grandad John (Nighy) lives.

On the voiceover, Finn explains his brother was born very prematurely, leaving him with long-term health conditions – so it’s a bit alarming when he shoves little Charlie in with the luggage on the coach at Manchester. Elsewhere along the journey, Finn has a wholesome encounter with a busker, played by Maisie Williams, who, in a film with a fair share of yucky moments, is forced to strum a ukulele and sing twee renditions of pop classics – including (inevitably) a couple of lines of the Proclaimers’ I’m Gonna Be (500 Miles).

The film is circling around something awful that happened in the past, which grandad John in Dingle blames himself for, and is responsible for a terrible family fallout. The tragedy slowly takes shape in flashbacks; kids in the audience may be shocked and moved by the twist that comes – but for grownups it takes this already unreal film into a dead end of emotional implausibility.