Every week nine-year-old Frankie gets a letter from some far-flung corner of the globe where his roaming sailor father has put into port.

Dutifully he writes back, telling pop how his devoted mum Lizzie (Mortimer) and irascible grandma Nell (Mary Riggans) are faring and how he's getting on at school.

It's an insecure life - as well as being deaf, Frankie finds himself flitting from Scottish town to town for reasons his mother won't reveal.

The reason is she walked out on his real father, taking him and his gran with her to dot around the country and stay one step ahead.

Lizzie has conjured up the fiction of a seafaring father to satisfy his curiosity as well as protect him from a past he's probably best off not knowing out.

However, the tangled web of well-meaning deceipt comes to a head when Lizzie "hires" Gerard Butler to act as Frankie's father for the day.

Debut director Shona Auerbach has crafted an affecting, low-key gem blessed with a sterling performance from Mortimer as the resourceful mum.

The actress demonstrated her grasp of the Scottish accent in Young Adam but here - in her biggest big screen role yet - she gives voice to a winning combination of doubt and certainty.

Set against the bleak landscape of post-industrial Clydeside, it's a story rich with subtle language and small, significant gestures.

Not a movie that shouts at you with Hollywood histrionics, but one that lets the action unfold at a steady, assured pace.

Thankfully, Auerbach doesn't tie things up in a neat bundle...but leaves loose ends hanging with the vague promise of something better.

Tim Evans