A stage set of a small Post Office branch might once have heralded a comedy of rural life or a romcom where a couple meet while sending love letters to others. Now, though, it portends a tragedy, the digital till on the counter a primed explosive.

Commissioned by the University of Reading’s law department and produced by inventive company Rabble Theatre, Zannah Kearns’s Glitch was written before Gwyneth Hughes’s gamechanging ITV four-parter Mr Bates vs The Post Office won publicity and the prospect of proper recompense for more than 700 subpostmasters falsely accused of fraud.

This means that the audience is well ahead of the characters for the first section of Glitch, which dramatises the story of widowed branch manager Pam Stubbs – a real-life victim from Berkshire – as she struggles with a till that seems to be possessed by an innumerate devil and Post Office bosses who seem more interested in wrecking her than helping her. These revelations are now reigniting rather than revelatory.

But the show sensibly makes fresher tracks, with Stubbs, a minor character in the ITV drama, maximised here. It explores her emotional backstory and how the paper-and-pencil note making for which she was denigrated by some became a crucial factor in exposing the scandal. This perspective makes Glitch a distinctive and original piece.

For budgetary but also artistic reasons, directors Gemma Colclough and Gareth Taylor eschew TV documentary realism for a more impressionistic, choreographic take. The cast – Fayez Bakhsh, Elizabeth Elvin, Sabina Netherclift, Laura Penneycard – switch without confusion from victims to villains, witnesses to lawyers, changing role by gestures as simple as a wedding ring removed or restored. Caitlin Abbott’s set – wooden boxes restacked or reversed to become shop, court, church hall, jail – should be taught at university as an example of theatrical economy.

Like the TV series, the play ends with Lord Justice Fraser’s 2019 judgment on the unreliability of the Horizon system and the questionability of Post Office evidence. With ITV ruling out a further drama, Kearns and Rabble seem well placed to fictionalise the ongoing public inquiry, perhaps with this superb quartet of actors alternating the tragic-comic evidence of former supremo Paula Vennells.

• At the Minghella theatre, Reading, until 6 July