The Boyle Family and I are sitting inside a tent, which itself is placed inside a huge, empty, slightly forbidding warehouse.

It may seem an odd location for a celebration but that is, in part, why we are gathering here.

This family of artists have produced their virtuoso re-rendered squares of the surface of the Earth for more than 40 years. They have been honoured with exhibitions around the world, and considered among the most important and singular (if that is the right term for a family working in concert) artists working in the UK since the 1960s.

The father of the family, the late Mark Boyle, was a Glaswegian poet and artist, whose perhaps most important work with his collaborator and wife, Joan Hills from Edinburgh, was made on the beach of Camber Sands in Sussex. If you attended the Scottish National Gallery of Modern Art's Boyle Family retrospective in 2003, you will have seen the large squares of re-created beach, displayed on the wall like paintings, resembling slabs of the ground as if removed by a huge cake-cutter. These pieces look incredibly real, inviting close inspection and an element of childlike wonder - prompting questions such as "How do they do that?"

But of course, these highly complex reincarnations of the Earth are in fact fabricated. Like all of their art, the beach works were made with a series of slightly mysterious processes which involve resin, fibreglass, sculpture and a degree of painting. Those particular 14 slabs of upended beach registered the different sand patterns created by the rising and retreating tides over 12-hourly intervals. The artists have gone on to make around 40 of a planned 1000 works, painstakingly done in a similar way, with plots of land from Italy to Norway, Japan to the coast of England and, in the future, Bute.

So why the celebration now? Over bottles of water and digestive biscuits, the family are happy to reveal that the Tidal Series now belongs to Scotland: it has been gifted to the country by the Peter Moores Foundation, which owned the series for many years. And it is part of a show opening at the Scottish National Gallery of Modern Art this weekend, which features recent acquisitions and gifts.

The Boyle Family's studio couldn't be more anonymous: a warehouse in a square mile of similar warehouses in a undistinguished area of south London, not far from Millwall FC's football ground. But I feel slightly honoured - journalists are not often welcomed into the family's work space. Today Joan (born in Edinburgh, 1931), Sebastian and Georgia (born in London in the early 1960s) are happy to show me around the functional space. The tent is in place, in fact, to add a degree of climate control to this huge area. Mark, who died in 2005, is perhaps present in spirit: his son says that the artist - originally from Pollokshields and who was celebrated for his psychedelic light shows in the 1960s London Undergound - would have been delighted by the turn of events. Sebastian, while not the leader of the family as such, speaks the most, while his mother chips in and his sister occasionally intervenes. On one wall is a new work, a 6ft by 6ft square of re-rendered reality, this time from a patch of burned ground near Lazio in Italy. Georgia allows me to touch it: it is rough and cold, hard and gravelly, rather like you'd imagine the ground itself to be. There are twigs and shells embedded in the earth. It seems real: but of course, it is not.

The Tidal Series was once shown at the Kelvinhall in Glasgow in 1973, to, the family notes, little interest at the time. "It is our most important completed series," Sebastian says. "We feel great about it. It's great that it's there. We love Scotland and it is really important to us that it is there, actually.

"And what we are hoping is that this show happens, and that other Scottish museums and galleries ask to borrow it. Quite a few of them now have benefited from the [Anthony D'Offay's] Artist Rooms project, and it could be a Scottish version of that. It would be great to see it in Orkney and Inverness and Falkirk. It is an artist room, in effect. But it is one work, it cannot be split up."

The size of the series means its displays have not been as frequent as could have been. Despite advances from galleries and collectors, the family have kept it together. "It is one work, and we kept hold of it for years and years, and we wouldn't split it, even though collectors and galleries wanted bits of it. We always wanted it to be one thing," Sebastian insists.

It was an exhausting process to make the Tidal Series. The area of sand chosen to copy - which changed with every tide - could only be marked by eyelines, landmarks and memory. And it had to be caught before it changed again. The work took all week. There are sweet photographs of young Georgia and Sebastian playing on the beach as their parents work. "What we had never worked out," adds Joan, "is that the tide changes every hour and a half, and if we going to do this twice a day, we were going to work through the night, which is what we did in the end. So we ended up living at the Kit Kat Cafe [on the beach] so we could make it."

"We feel we have really built a really strong relationship with the SNGMA," says Sebastian. "We're delighted: we really like them and they seem to like us." He and Joan speak of the "freedom" that Mark Boyle felt when he moved from Scotland to London in the 1960s.

But, Sebastian adds, "Scotland is just transformed now. Especially Glasgow, but also Dundee and Edinburgh, and especially with the visual arts and the arts in general - carving out a separate identity for itself away from England. That is a very important thing for it to do. It is good that is happening."

Mark and Joan met in 1959 and produced work together from that moment (in a Harrogate coffee shop). In the early 1960s, they became involved in theatre and performance art, as well as contemporary art. When their children arrived, they naturally became part of the process, and since the 1980s have been known as the Boyle Family. Cameron Hills (the "fifth Boyle"), Joan's child who was born before she met Mark, also collaborated for several years. Georgia and Sebastian say that no specific task in making the works is delineated. Arguments happen over how something can be made, they both acknowledge, but usually an agreement is reached.

What is never changed is the method of making: a right angle is thrown in the air, and lands on the earth. That right angle is the first corner of the square to be made. Nothing inside the square of earth is changed or adapted or made "easier" or more attractive.

It is a rigorously objective process. Mark Boyle once said: "As far as I can be sure, there is nothing of me in there." Georgia adds: "It is absolutely vital that we don't move anything: that is why we call it contemporary archaeology, and actually anything you find [on the ground] is fascinating. And, anyway, who would you be changing it for?" The works come with studies of the plant and insect life inside the square. It is, they say, all about the depicting reality.

The works are essentially reliefs, and in are hollow and relatively light. Colours are made from crushed rocks and other natural sources; instead of canvas, there is fibreglass. Rock, mud, earth, drain covers, a garden hose, a gym shoe, sand are all painted and hollow. The family usually work on three or four pieces simultaneously, making perhaps 10 or 12 large pieces a year.

"It is about trying to tell the truth," says Sebastian Boyle, "and it is about being as accurate as you can in the telling of that truth. We think the more accurate you can be, the more interesting it is. Lots of people say they are doing art for themselves - but we actually are. And we are the only ones that can check it is real."

New Acquisitions is now open in Modern One at the National Gallery of Modern Art, Edinburgh,