Might knob gags be defended on the basis of cultural inclusion? Frank Skinner claims they were the primary means of communication where he grew up, as indelibly a part of him as his Black Country brogue. OK, so comedy has changed, and he can’t be so fast and loose with the filth as he once was. But – as he reports in 30 Years of Dirt, in an image for the ages – Skinner can no sooner extirpate knob gags from his imagination than the ghost of Cathy Linton be stopped from tapping on the windows of Wuthering Heights. By the end of this Edinburgh hour, then, 30 Years offers itself as a celebration of smut. But it’s smut without crudeness; gentlemanly smut, even, from a 66-year-old with a poetry podcast. And it’s not just smut: there’s a meandering buildup to that section of the show, as this fringe veteran takes in his august surroundings at the Church of Scotland HQ (“it’s all a bit Protestant for me …”), banters with the crowd, and deploys random anecdotes and jokes from what, he gives every impression, must be an inexhaustible supply. There’s a misunderstanding about composer Tim Rice to make the mind reel, a groansome Nostradamus gag, and much mockery of Skinner’s status as “a once-great comedian”. Is Frank stretching himself here? Hardly. But then, his laconic style is deceptive: Skinner is never less than beadily alert, and the quality of the jokes – and of his company – more than makes up. A routine about the blowjob dying out reaches its apotheosis with an explosively ridiculous line about Gandhi. A section on his youthful experience working with foul-mouthed women in a metals factory turns everyday bawdy into overblown, looking-at-the-stars poetry. He even redeems my least favourite genus of standup routine, the hobnobbing with royalty anecdote, steering his Wills-and-Kate tale far from reverence and towards something unexpected and surreal. You come away happy that after 30 years Frank’s still dishing his dirt, and happier still that he can dish plenty more besides. At Assembly George Square, Edinburgh, until 27 August All our Edinburgh festival reviews