Nor indeed does his Peugeot van as he tears around a series of blind corners shaded by primrose hedges. “If a person’s worth commemorating,” says Frank, with is foot on the accelerator, “it needs to be an article of quality.”

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Doubling as their place of work, the house on the river is instantly recognisable as a home to artists – a large piece of stained glass restoration work rests before the porch. In every nook, there’s all manner of tools and moulds. There are hints everywhere of the active life, from muddied wellies and fishermen’s overcoats, to experimental pottery and clay work.

The family started by taking on private commissions and continues to make them today – particularly close to Frank’s heart is the plaque commemorating the victims of the 1981 New Cross house fire. Frank may now be a full-time craftsman, he says, but he began his career in the arts relatively late. A quantity surveyor for 12 years, he worked in Tehran and Kuwait following his two years of national service between 1949 and 1951. Studying design at Goldsmith’s, where he met his future wife Sue, he quickly moved from painting to ceramics.

^{Alfred Hitchcock’s blue plaque at 153 Cromwell Road (Alamy)}

Commemorating London’s most cherished cultural and historical icons, blue plaques first appeared in the capital as part of a Society of Arts initiative in 1866 commemorating the births and deaths of people as varied as Handel, Hendrix, Hitchcock, Woolf, Plath, Lennon, Pankhurst and Wren. It was one year before the plaque dedicated to Lord Byron (long since demolished) made its dark blue debut in 1867. Since then, the scheme has changed hands three times, before being taken over by the English Heritage in 1986.

In much the same way that the scheme has changed hands, so has the production of the ceramics. “The plaques give nothing away – they’re poker-faced,” says Justin, heir to the family business. “With such few words they only hint at the mystery of a hidden life.” That’s perhaps true not only of the wording, but also of the craft itself.

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The plaque-making process comprises three main elements: the casting of the special clay in a plaster mould, its lettering, and the cobalt blue glazing. Each element is a painstaking process in its own right and was the result first of patient experimentation then years of dedication and craft to perfect. There’s a bit of luck involved as well – every step requires a stoicism and at times blind acceptance that all simply won’t go to plan.

It begins with a kind of artful alchemy weighing the ingredients for the clay mixture. “Weeks of concentration and labour can be ruined seemingly without reason – even a small crack during firing means we have to start again,” says Frank as he handles the plaster mould which forms the ceramic base. “That’s the nature of working with these materials.”

^{Virginia Woolf’s blue plaque at 29 Fitzroy Square(Alamy/REX)}

Based on a time-honoured and carefully weighted blend of ball clays, sand, felspar and an aggregate grog known as molochite produced within a few miles from the studio, Frank says this particular mixture ensures durability, and it forms the basis that enables its famous blue glaze to shine brightly. “Without a mixture like this, the colours would change once applied to the plaque,” he says. “The whites and blues would become tinted and altered with another kind of clay mixture beneath.” They pour the mixture into a specially made plaster mould and fit a separate back mould to form ribs and a strong perimeter, all concealed upon installation.

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But there’s one important difference to the original recipe that subverts tradition. “We use a secret ingredient in the clay to make this process possible,” says Frank with a grin, before Sue buzzes in to divert the obvious next question. “A fired stoneware plaque of these dimensions and structural design are incredibly strong and long-lasting. It’s made this way for precisely that reason.”

The clay is left to dry untouched for around three weeks before work can continue. Cups of tea are drunk, yoghurts devoured and other work resumed. Sue, for one, restores glasswork on commission, while Justin, once head boy of the Queen’s choir, is a jobbing musician back in Blackheath, southeast London. Frank might be found doing any number of solo pottery projects – bringing an entirely new meaning in his unique way to pottering about the garden.

Next comes the text for the plaque. As Justin raises the 16kg base, he says: “This will be the same in 200 years’ time. It’ll be stronger than the building itself.” He lowers the blank leather hard base on a turntable and begins the protracted process of lettering. First, he outlines the letters lightly using a guide. Tube-lining is the steady-handed method the Ashworths use to shape the names, dates and letters on the readied blank.

^{(Christopher Jones)}

For Sue, this method reaches back in time to calligraphic tradition. “The lettering itself is the original used by an early sign maker and plaque designer, Henry Hooper, which is a modified version of the classic Roman lettering. We digitally designed the letters to fit how Hooper first drew them.” It’s the only digitised part of the process, says Sue, and the reason for its use was a way of preserving that style. “This way of making the plaques doesn’t just honour the person, it also honours that process.”

The tube-lined letters, after drying, are then “biscuit fired” in the kiln to solidify the text. After a few days of cooling (and more yoghurts) the glaze is weighed and readied. “The blend of materials we use for the blue is roughly the same as glass. We use a slip trailer to extrude it out, but the consistency will be poor if you’re not careful. You’ve got to finish it with a thickness of glaze which is just right,” says Frank. “If it’s too thin the colour will change, and if it’s too thick it will be lumpy.”

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Sue leans in: “It’s very difficult to tell at that stage whether you’ve filled all the right areas correctly, because the glaze goes on in liquid form with the consistency of single cream, plus it’s the colour of the palest pink. If you make a mistake, it could cost days more of work. You have to look for trouble.”

All this means there’s a palpable sense of portent as the clay is delicately worked for every stage. The temperature in the kiln is yet another crucial element to success. Getting it wrong could mean the worst: a crack, and a lost plaque. “We may have done 304 plaques officially, but that’s not counting the ones that have for whatever reason gone wrong,” confides Frank as he takes the next bend at speed toward Lostwithiel train station. “The real number is rather more than that.”

It will still be days or maybe weeks before Georgette is ready to reluctantly receive her public. In the meantime, there’ll be more tea and sandwiches, small skips across the river to town – and a lot more work. For now, it’s waiting. And just as we depart, Frank tells me another commission has come in to their London Plaques inbox, which he’ll begin work on when he gets home. Whoever it might be, I think, they couldn’t be in safer hands.