Bowling up to the world's largest arts festival to perform in the style of a radio-play recording is an audacious move: surrounded by experimental dramaturgies, passionate autobiographical scripts and ambitious comedians, Fags, Bags and Mags has a gentle humour, a limited sense of theatricality, and relies on recognition of the characters from its presence on Radio 4. Sadly, in a live setting and stretched beyond the short form of the sit-com, its script is exposed as the weak comedy mocked by Ricky Gervais in Extras, complete with characters making strange noises for no apparent reason.

If the format – actors reading from a script, often as if they haven't seen the words before – is limiting, some lazy performances and poor characterisation takes Fags, Mags and Bags into a new domain of disappointment. The characters are barely developed, the pop-culture references dated – the odd stab at instagram jokes are desperate attempts to drag the formulae out of the past – and the ensemble cast take on multiple roles that are little more than fodder for a cheap joke. Even the main characters are wafer-thin, and single ideas (one son looks a bit like someone from TV, another has silly business ideas) are dragged out. Audience walk-outs are frequent, and even the goodwill for Sanjeev Kohli isn't enough to rescue the production.

On radio, Fags, Bags and Mags is vaguely distracting, but as a live show it is revealed as a dismal throwback to safe, predictable comedy that relies on stock characters – and jokes about the stock of a shop – to drive forward an insipid plot and set up naive comedic routines. Despite a brief interlude with a prospective MSP saying the wrong things (mostly homophobic slurs), there is little to suggest any engagement with the twenty-first century, and the description of Lenzie, a suburb of Glasgow, is a mixture of cliches and tropes about the oddness of suburban life. Some of the cast are lively, others are phoning in their contribution, and the lack of care in the production suggest a slap-dash and cynical attitude towards what the audience wants.

Underbelly George Square, until 26 Aug, 4.40pm, £13.50 (£12.50).