Tate Britain has spent five years bringing together some of the greatest pre-Raphaelite works for a show that repositions the artists as the radicals of their day. We witness the culmination of a huge project, as everything, from the largest Burne-Jones to the smallest fridge magnet, finds its place…

In a huge house in a mysterious part of London, a tall, energetic man called Rupert Maas is showing me a drawing: The Lady of Shalott by Elizabeth Siddal. "It's absolutely lovely, isn't it?" he asks, though I have the strong impression that he doesn't give two figs whether or not I agree with him. "There are no more ethereal drawings produced by any of the pre-Raphaelites than those by Lizzie, and this is a very, very good one." His voice runs on: not dreamily, exactly, but clotted with a certain kind of passion. "It has this febrile intensity. It's deeply sexy, for some reason. Look at the tightness of her dress, the yearning quality of it." Somewhat trepidatiously, I tell him that, to me, this particular Lady of Shalott – Lord Tennyson's Arthurian maiden, condemned forever to see Camelot only in the reflection of a mirror, was a favourite subject of the pre-Raphaelites – looks a little like a doll. "Yes, well... I think that might be part of it," he says, with a smile.

Elizabeth Siddal, the redhead who is perhaps best known as the model for John Everett Millais's Ophelia, was married to Dante Gabriel Rossetti, one of the seven founders of the pre-Raphaelite Brotherhood, from 1860 until her death from an overdose of laudanum in 1862 – though their relationship began in about 1851, when she first started sitting for him. "The relationship between Rossetti and Lizzie is absolutely central to the pre-Raphaelite spirit," says Maas, still peering over my right shoulder. "She is Beatrice to his Dante." But Siddal, who had humble roots and had previously worked as a milliner, also longed to be an artist in her own right; in 1855, John Ruskin agreed to subsidise her career, paying her £150 a year in exchange for every drawing she produced. "It's well-documented," says Maas. "He [Rossetti] taught her. He stood over her while she drew, and he did bits that she couldn't manage. It was a thing they did together: a journey of love into another world; a medieval paradise for them both." Did Rossetti work on The Lady of Shalott, which is dated 1853? "I think he might have had something to do with the sprite carved on the chair. I think they did that together."

In the next few days, The Lady of Shalott will depart this house for Tate Britain, where she is to appear in the gallery's autumn blockbuster, Pre-Raphaelites: Victorian Avant-Garde. There, it will join works owned by, among others, Jimmy Page and Andrew Lloyd Webber. Is it thrilling to lend to such an august institution? Not for Maas. This is the second time Siddal's drawing has holidayed at the Tate (the first was during the gallery's last pre-Raphaelite show in 1984), and there are other, equally wonderful works in his collection. "I do it all the time, quite honestly," he says. "I think it's a public duty. When it's there, I won't even look at it; I'll go and look at something I'm not familiar with instead." He grins. "But, of course, when it comes home, I'll have a jolly good gloat."

Maas inherited some of his collection from his father, Jeremy, an art dealer who in 1969 wrote a celebrated book about 19th-century British art, Victorian Painters, and who began buying 19th-century British paintings when they were still amazingly affordable. (Rupert now runs his father's Mayfair gallery.) The rest, he bought: "I'm not one of those dealers who feels he shouldn't collect." So what is it about Victorian art in general, and the pre-Raphaelites in particular, that speaks to him? They haven't always, it's fair to say, been terribly fashionable.

"Yes. But when people say they hate Victorian art, you have to ask: what is it they're hating? They're hating themselves, because they're hating the stuff of which we're made. Most middle-class people in Britain still live in Victorian houses. They gave us all sorts of things we take for granted. And Victorian genre paintings deal with such serious social issues. Look at Ford Madox Brown's The Last of England [a painting about emigration and poverty that will also be in the Tate's exhibition]. He's asking big questions in pictures."

As for the pre-Raphaelites proper, with their penchant for swooning damozels and complicated allegories, he hopes that the Tate's vast new show will persuade visitors to reconsider them. The pre-Raphaelite Brotherhood, formed in 1848, was determined to rebel against dreary Royal Academy conventions; for this reason its members have sometimes been compared, in spirit, to the Young British Artists of the 1990s. Maas, though, likens them to punks; every young artist wanted to be one. "Millais was the greatest draughtsman. Rossetti was the romantic, the natural heir to Blake. Holman Hunt is more difficult: the priggishness, the religiosity, the density: these are some seriously wacky paintings. But they're all so big, so brightly-coloured, so powerful. You can just imagine how they must have seemed once, when everyone was used to seeing Sir Sloshua Reynolds and his school." His eyes widen. "They must have seemed seriously psychedelic."

It has taken Alison Smith, a Tate curator, more than five years to put Pre-Raphaelites: Victorian Avant-Garde together. The idea for it came to her after the Tate's Millais exhibition in 2007. It encouraged visitors who thought of the artist as a painter of fancy chocolate-box pictures to see him in a different, more audacious light, and Smith found herself wondering if she couldn't do the same for his colleagues in the pre-Raphaelite Brotherhood. "I wanted to show them as modern artists rather than as soft romantics," she says. "That was my agenda."

Her case successfully argued (the Tate's programme is driven not by potential visitor numbers but by intellectual inquiry, with the result that every show must have a thesis), and the exhibition safely in the schedule, she began work. The Tate has a peerless collection of pre-Raphaelite paintings, among them Ophelia by Millais, The Golden Stairs by Burne-Jones, The Beloved by Rossetti, and The Triumph of the Innocents by Holman Hunt. But she also had a list of must-haves to be borrowed from elsewhere: Millais's Isabella, which hangs in the Walker Art Gallery in Liverpool; Holman Hunt's Rienzi Vowing to Obtain Justice for the Death of His Young Brother, which is in a private collection and had not been seen in public since 1984; Ford Madox Brown's Work from Manchester City Galleries; Holman Hunt's The Light of the World, from Keble College, Oxford; and, most fabulous of all, Holman Hunt's The Lady of Shalott, from the collection of the Wadsworth Atheneum Museum of Art in Hartford, Connecticut. This massive painting, overpowering and, some would say, overwrought, has not been seen in Britain since 1951.

So how did she do? Lizzie Siddal's drawing, just across the river, must have been relatively easy to clinch. But what about the others? "Oh, we secured them all," she says, almost nonchalantly. "Everything we wanted, we got. The Lady of Shalott was in doubt for a while; there were conservation issues, and we had concerns about travel and costs. But, in the end, all the problems were resolved, and this astonishing late painting [it was completed in 1905 after the artist's death by an assistant, by which time its subject was already long out of fashion] will be the final work in the exhibition." And once the names on her list were ticked off, did she dance a little jig? "No. But you feel good for the show. You feel it's finally coming together."

Its major paintings bagged, the exhibition began to spread its tentacles outwards. All hands on deck. Backstage in London, Kiko Noda, the show's registrar, embarked on the complex logistical task of arranging the transportation of every loan. "Most lenders insist on a representative being present when a painting is hung," says Smith. "And once a work has been hung it cannot be moved. You can't go back and say, 'Oh, that would look nicer there.' A big lender might have five or six works in different rooms, so drawing up an installation schedule is perhaps the trickiest thing of all. That's where Kiko comes in."

In America, Smith's co-curators, Tim Barringer and Jason Rosenfeld (the exhibition will travel to Washington), started working on their scholarly essays for the catalogue. At Tate Enterprises, the team began thinking about merchandise: scarves inspired by the gown Rossetti's model wears in The Beloved; bags and cushions made from fabric designed by the pre-Raphaelites' friend and supporter, William Morris; and, of course, fridge magnets and postcards, which sell in their thousands. (Tate Enterprises earns between £2m and £3m a year for the galleries, so ordering the right merchandise is a serious business.) And the marketing department considered how best to attract younger visitors. Among their ideas: pre-Raphaelite-inspired fashion shoots; a pre-Raphaelite Pinterest page; a roll call of "modern day muses" with pre-Raphaelite sensibilities (Paloma Faith; Florence Welch, from Florence and the Machine).

The months, and the years, ticked by. It's now August and the paintings are finally arriving; Kiko Noda receives every one personally. They will be hung by a team led by Geoff Hoskins, a senior art handling technician of 20 years' experience, in the fortnight before the show opens on 12 September (the wall texts were completed only in the past few days – Smith's American colleagues slaved through the night to finish them on time). What will it be like to see the work in the galleries at last? Smith smiles. "For me, the most wonderful moment is installation. It's the culmination of everything. That's when you feel you are deep in the heart of a project." And when it opens to the public? "The personal attachment loosens a bit, but you're still concerned. It's a like a child going out into the world: you want it to do well."

The Tate's pre-Raphaelite paintings are among its most popular (Ophelia by Millais, so lush and yet so plangent, has long been the gallery's bestselling postcard). "They're always on display," says Natasha Walker, a paintings conservator. "And when they're not, they're often on loan to another gallery. So it's quite rare that we get the opportunity to look at them. That's why we like these big shows. It gives us the chance to get our hands on things."

Some time ago, Walker and the other conservators examined all of the Tate paintings that will appear in the exhibition. "We have priorities," she says. "Obviously, if something is stucturally unsound, that's the first priority. This one [she reaches for some images] was displayable, but we wanted it to look its best. So I spent five months cleaning it."

The painting in question is Dante Gabriel Rossetti's Beata Beatrix (1864-70), a portrait of the poet Dante's wife, Beatrice, that was also a memorial to Rossetti's wife, Elizabeth Siddall (the painter used sketches of his late wife to complete it). It's an exquisite piece, smaller and more intimate in scale than many pre-Raphaelite works, and softer, too, being more dimly lit. Beatrice/Siddal has her eyes closed, though whether she is in a state of religious or sexual ecstasy is unclear. Meanwhile, a bird, a messenger of death whose feathers are the colour of dried blood, drops a poppy into her open palms (this must, surely, be a reference to Siddal's death from an overdose of laudanum shortly after she gave birth to a stillborn child). In the background is Dante, looking towards the haloed figure of Love, in whose hands the burning heart of his wife flickers and wanes.

"The painting had a natural resin varnish over it," says Walker. "It would have been added to improve its colour rendition, the gloss of it. But it had become quite discoloured." After doing some tests to find which mixture of solvents would best remove this layer, she set to work with a cotton wool swab. "It's very painstaking. You have to be careful with the paint layer." She shows me some before and after pictures. "Look at the colour shift. Before, it was warmer and quite yellow in tone. Varnish tends to make things look quite unified. The contrast between highlights and shadows is so much greater now, and her flesh is cooler, not quite so glowing."

During the conservation, Walker x-rayed the picture; she also photographed it while casting light at an acute angle over it. "I found out quite a lot. The story goes that Rossetti had made and abandoned an oil sketch of his wife, and that it lay in his studio for many years, until his dealer took it to be adhered to another canvas, and brought it back for Rossetti to finish. The x-ray showed that there were indeed canvas additions at the top, sides and bottom, all of which would have allowed Rossetti more scope for background." She shows me the x-ray. "I could also see these losses in the lead white preparation under the paint. Rossetti left these losses. A more meticulous artist would have filled them before recommencing. When I cast light over the picture, I could also see brush hairs, studio dust and debris in the paint, which tells you something about the state of his tools."

It took Rossetti six years to complete Beata Beatrix, a long time for a painting of this size. "When I looked at the green of her cloak I could see that it had aged and cracked over a period of time; his red monogram had been added over the cracks. I could even see some of the red pigment caught up in the varnish, which tells me that he signed it and then quickly sent it away."

How do these discoveries make her feel? Shivery, is the answer – though in a good way. "You've seen it in books, or on walls. But this brings you so much closer." It's a visceral thing, a connection with the artist himself. What do her discoveries tell us about Rossetti's state of mind? Walker is reluctant to say: "I'm not an art historian," she laughs (her degree is in zoology, the chemistry she learnt then a great help). But to me it seems obvious. Rossetti took his wife's death hard, burying the bulk of his unpublished poems with her in Highgate cemetery. Afterwards, he grew increasingly depressed. Beata Beatrix was a painful piece to paint; it took him an age. But when he finally felt able to let it go, he couldn't wait to get it out of his sight.

It's not only paintings that must be conserved. When Walker and I have finished talking, she takes me to the studio of Alastair Johnson, a frame conservator. For the past year, Johnson has been working on the frame of Burne-Jones's enormous oil, King Cophetua and the Beggar Maid – a project that will be completed any day now, when picture and frame are once again reunited.

King Cophetua was completed in 1884. It was inspired by a Tennyson poem, The Beggar Maid, and tells the legend of an African king who disdained women until he met Penelophon, "a beggar maid all in grey", with whom he fell in love at first sight. In the painting, the king gazes at Penelophon devotedly (the theme of male enthralment to female beauty was a favourite of Burne-Jones's), having put aside his crown and shield in deference to her beauty. A deeply sensual work – with her gently rounded belly and her curled toes, there is something so straightforwardly sexy about Penelophon – Burne-Jones's friends bought it for the Tate after the artist's death in 1898.

The painting's frame was made for it, in the Renaissance style, by the Vacani family. But a few years after it joined the collection it was altered to accommodate a glazing door (in other words, an ugly sub frame was fitted inside the original one). "The alterations were substantial," says Johnson. "They had to insert four inches of material in the bottom of the frame, removing one putto's head, and replacing it with two. They also built up the columns at the side and removed altogether the frame's lovely moulded [internal] edge."

How did Johnson know what the original frame looked like? Luckily, there existed a photograph by Emery Walker of the painting in its original frame; Johnson found it at the National Portrait Gallery, where Walker's archive is kept. Using this as a guide, he set to work. First he removed the additions. "They were quite brutal," he says. "You could see the saw marks where they'd cut the bottom of the frame in two." Then he made moulds of a putto's head elsewhere on the frame. "I used a silicon rubber mould; prop-makers use them; they're incredibly detailed. The heads themselves are made from composition, a doughy mixture of plaster or chalk." Then he set about copying the moulded edge. "Usually we find another frame which has something similar as a guide. But I couldn't find one anywhere. In the end, I just modelled a section up in Plasticine."

Finally he guilded his repairs. Johnson leads me to the frame itself, in the centre of the room. Wow. What a wonderful thing it is. But will he age his own additions? They're a bit bling at the moment. "Yes. I'll probably use watercolour: something I can easily remove, or add to. And perhaps a bit of household dirt." It will, he says, be an anxious moment when the painting is put into the frame. "A very expensive piece of Perspex – it's called Optium – will replace the glass. It has an innovative coating which makes it look like low-reflective glass. It also has an inherent flexibility, which is important with a painting of this size. Unlike glass, it won't crack." How will he feel when he sees it in the gallery? "Oh, it'll be wonderful," he says, with great feeling. "But I'll also be praying I measured the painting correctly."

As both Walker and Johnson point out, thanks to their efforts, when visitors to the exhibition come to gaze on Beata Beatrix and King Cophetua in a few weeks' time, they'll be seeing them pretty much as Rossetti, Burne-Jones and their contemporaries saw them. Will this make a difference to their understanding of these artists? Perhaps not. Only experts and passionate fans will notice such subtle changes. On the other hand, as the late, great Robert Hughes put it, for the pre-Raphaelites, "God was in the details: in the petals of a cornflower or the vein of an elecampane leaf, in the grain of stone or the purling of a brook."

Rossetti and his friends would, I think, have adored the care the Tate has put into this show. Such attention, loving and precise, reflects the extreme trouble they went to in their own pursuit of accuracy. Though what they would have made of fridge magnets is anyone's guess.

Pre-Raphaelites: Victorian Avant-Garde is at Tate Britain, London SW1 from 12 Sept to 13 Jan 2013; tate.org.uk