This year’s first intercutting monologue play is David Harrower’s A Slow Air, a beautifully written but finally flaccid (and sometimes hard to follow) study of an estranged brother and sister played by real-life brother and sister Lewis Howden and Kathryn Howden.

These fine actors, directed by the author, explain themselves but, as always in these types of plays, you wonder why. There’s none of the theatrical throb of Harrower’s Knives in Hens, or revelatory mounting tension of his Blackbird.

Athol, Morna’s elder brother, is the owner of a floor-tiling company near Glasgow Airport, where the terrorist attack of 2007 comes into play. Morna works as a cleaner in Edinburgh, living with her grown-up son, spending time in pubs with unsuitable prospective lovers.

The most enjoyable part of the play, for a festival audience at least, is the topography of the city, which comes alive in Morna’s descriptions. The 14-year-old stand-off starts to melt once the son visits Athol, and family wounds and differences are worked through their bitter memories.

So much of the piece, first seen in May at the Tron in Glasgow, is good writing but slow theatre, and even at eighty minutes it seems unconsciously long. But it’s all done with a steady air of decorum, minimally designed by Jessica Brettle and nicely lit by Dave Shea.