In later life, my maternal grandfather was fond of recalling a specific spring evening he’d spent as an Italian POW in South Wales. It was 1945 and, after a hard(ish) day’s labour at a local dairy farm, he was taken for an unofficial drink in a Newquay boozer. There, he was introduced to a proud but personable poet named Dylan Thomas, who, over a pint or three of stout, questioned him about the ample merits of Mediterranean women.

It was a tale I never quite believed: there were probably any number of would-be poets called Dylan in Welsh pubs at the time. Yet, it certainly has the ring of truth: revolving around Thomas’s legendary liking for ladies and drink. Indeed, one of the core aims of DT100, a Wales-wide celebration of the centenary of Thomas’s birth next year, will be to shift focus away from the life, and back onto the work, of this great 20th century writer.

Kicking off DT100 early, National Museum Cardiff is hosting an exhibition by pop art pioneer Peter Blake of images inspired by Thomas’s most famous work, the radio play Under Milk Wood. Blake has been working on the series, off and on, for 28 years. A keen fan, he remembers hearing the play’s very first broadcast on the BBC in 1954, six months after its completion and just two months after Thomas’s death in New York, aged 39, purportedly after downing 18 whiskies in quick succession at the Chelsea Hotel.

Under Milk Wood recounts a day in the life of the imaginary Welsh fishing village, Llareggub. Some 60 colourful characters feature – from widowed Mrs Ogmore-Pritchard, henpecking her two husbands beyond the grave, to Mr Willy Nilly, the nosey postman who steams open the locals’ mail.

For the show’s first room, Blake has created graphite pencil portraits of each character – utterly unsuccessfully. In part, this is because Blake didn’t conceive them from scratch himself, but – in true Pop Art fashion – modified existing imagery from books and magazines. In some cases, his inspirations are clearly recognisable celebrities: Beryl Bainbridge for Mrs Utah Watkins, Elizabeth Taylor for Rosie Probert, and (yes, seriously) Terry Wogan in drag for Mrs Waldo.

It’s hard to know, really, who these portraits are intended for. They’ll hardly appeal to the uninitiated, while the play’s devotees are unlikely to change the image of these characters we’ve mentally cultivated for years. (This is a problem, of course, for any depictions of a beloved piece of literature appearing after its publication – in contrast to, say, Tenniel’s illustrations for Alice in Wonderland, which appeared simultaneously with it.)

Blake’s watercolours of the characters’ dream sequences, which open the play, are more successful: painted in bright, hyper-realistic colour in a medium whose fluidity matches perfectly the transience and insubstantiality of dreams. We witness Mrs Willy Nilly as a naughty school girl, getting her buttocks spanked for being late; blind, old Captain Cat remembering his lost seamates on S.S. Kidwelly; and, most surreally, Butcher Beynon riding on a pig’s back, shooting down flying giblets.

I sometimes feel sorry for Blake. A Royal College of Art peer of Auerbach and Kossoff’s, he’s 81 now, his estimable career bestriding seven decades. Yet, to the public at large he’s only really known for designing the Sgt. Pepper album cover for the Beatles. To rub salt into the wounds, his dealer signed away his right to royalties for that work for a derisory upfront fee of £200.

Will Under Milk Wood help release the albatross from his neck? In a word, no. The exhibition’s largest section is devoted to scenes of Llareggub’s locals going about their daily business, and here Blake opts for collage. The best examples feature photos of the Carmarthenshire town of Laugharne (Thomas’s home and main inspiration for Llareggub), which add an authentic richness.

In the main, though, Blake wrongly equates the odd goings-on in Under Milk Wood with complete fabrication on the writer’s part – and therefore apt for a madcap mash-up. Thus, young Mae Rose-Cottage lying naked among nanny goats in a field of clover is rendered as a Playboy pin-up, amidst goats from a natural history textbook, against a flat green backdrop.

Yet, as anyone will know who has visited Welsh coastal villages even in recent years, Llareggub is rooted in reality – albeit of a heightened kind – and was intended simultaneously as both a parody of, and ode to, such settlements.

In other images, Blake tries and fails to match the text’s richness – simply jumbling components together in one over-literal picture, where Thomas had carefully constructed clause upon clause, often using that crazy device called a metaphor. How, for instance, to depict the “titbits and topsyturvies, bobs and button-tops, ash and rind... the moulted feathers of dreams, wrecks and sprats and shells and fishbones, whale juice and moonshine” washed to shore? Answer: with difficulty.

Which points towards the key flaw in this whole enterprise. Thomas stressed Under Milk Wood was a “play for voices”: for radio not theatre. His hypnotic Welsh cadences – most famously, in the lilting pre-dawn description of the “sloeblack, slow, black, crowblack, fishingboat-bobbing sea” – reflected his belief that the sound of words mattered as much as their meaning. (As Robert Graves said of Thomas, “Drunk with melody, what the words were he cared not”.) Here was a work intended to be heard not seen, and therein lies the problem facing any artist attempting to depict it.

DT100 promises an exciting toast to all things Dylan Thomas – I, though, shan’t be charging my glass just yet.