There’s something undeniably charming about the inimitable Gert Dumbar – grinning from ear to ear – referring to Andy Altmann and David Ellis, his former students, as “darlings of mine.” Or Phil Baines, his face lighting up as he delightedly recounts a story about how, in response to a brief set at the Royal College of Art, the friends put on a school play about internal combustion engines. Or how Margret Calvert speaks about the pair with genuine fondness despite, at times, giving away hints that the task of tutoring the group might have been… wearing.

‘The why nots’, as Altmann and Ellis’ contemporaries, collaborators (or, perhaps co-conspirators) and former tutors all refer to them, sound like the boys that might have lived down the road from you growing up. Schoolboy-ishly naughty. Charming, though, and affectionately remembered by everyone who encountered them. And really, wnax30 – the retrospective exhibition that runs at Sheffield Institute of Arts until December 22 – is about them more so than the work, the personable-ness that came out in their practice, and the working relationship that has lasted for three decades. Because there’s something genuinely likeable about ‘the why nots’, the lads who – along with co-founder Howard Greenhalgh, suited and professionalised under the title why not associates – walked into the bank for a business loan to start a studio, and then giggled about it later in the pub, astonished that they had really pulled it off.

Three decades of work followed, joyfully disregarding the kind of Graphic Design (big G, big D) that takes itself too seriously. “I always have this desire to scribble on things…”, Altmann says, “I think it’s because its the human side of something.” Nothing was considered precious, untouchable. On display are pieces of early work, traditional graphic design craftsmanship and newly available computer programmes mashing together, the visual workings out of how these two worlds, irreconcilable at the time, could be combined to create something new. (Steven Heller would disparagingly describe the work as part of the ‘cult of the ugly’.) Scamps for books, layered with post-it notes to printers and handwritten memos like ‘ANDY CALL ROCCO’ or fax numbers, revealing the journey from initial ideas to the final, beautifully printed book that sits alongside. Even discarded stone letterforms, carved from a larger – too large, it turns out – piece of work, become desirable objects in their own right. (I’d forgotten, until I saw them decorating vitrines, that I have one of the letters that were jaggedly cut out of the Comedy Carpet and sold off sat on my windowsill at home.)

And although you can tell a lot about the pair from the work, the most interesting stories are told by the people who were there. Pam Bowman and Matt Edgar’s documentary [trailer shown above], on show as part of the exhibition and set for wider release next year, interviews friends and colleagues – including Jonathan Barnbrook, Howard Greenhalgh, and Rick Poynor, as well as Dumbar, Baines and Calvert – who are quite candidly revealing about the pair and their working process. Their ability to be challenging, combined with a generosity and brilliant sense of humour, made them different to other studios. “They’re the classic British double act,” says Barnbrook.

None are quite as candid, though, as Altmann and Ellis themselves, in real life as well as on camera. On the exhibition’s opening night, it was David who spoke about his time working with Andy with such reverence that everyone – Altmann included – struggled to contain their emotion. Afterwards, a massive slab of white cake was presented to the pair, edged with frilly icing and a giant ’30’ on top. And of course it was cut by both of them. Because it isn’t why not associates’ – the studio’s – 30th birthday; it’s the 30 year anniversary of ‘the why nots’ themselves.

“It means it can be done. It’s like a band that stays together forever,” says Browns’ Jonathan Ellery about the studio’s longevity, “because they don’t, do they?”

wnax30 is at Sheffield Institute of Arts until the December 22. Details here