'I am still a wee boy at heart: and yes, all this is pretty cool'

The 49-year-old Glaswegian, whose equine and figurative sculpture adorns public spaces from Australia to Leeds, from Cumbernauld to Chicago, has his biggest work yet – perhaps ever – being constructed as you read this. His two enormous Kelpies at the Helix, Falkirk, gigantic rearing horses heads, are 100ft tall and weigh more than 300 tonnes. The heads, based on two horses in Pollok Park, Glasgow, will dominate their landscape for years to come, and will, perhaps, be the sculptor's biggest contribution to the public art realm.

Unlike many of his fellow, more critically-lauded graduates of Glasgow School of Art, Scott's work occupies the public realm – his clients are often public bodies and developers, his ready audiences not art fans in white-walled galleries, but tens of thousands of commuters in cars and members of new or re-developed communities.

His practice is one that involves a certain degree of self-restraint, and although he has plenty of "wild and fanciful" ideas, the strictures of public-place art involve a constant series of checks, balances and the finding of middle ground. It is a world of pragmatic choices, diverse client groups, tendering processes, logistics, regeneration projects and the demands and restrictions of structural engineering.

Still, in his output, the sculptor tries to imbue some individual meaning, often in semi-hidden signs and symbols, and his own artistic vision. Compromise, he says, is often the name of the game, or as he likes to call it "measured restraint".

The design and construction of the Kelpies has taken more than six years and are Scott's biggest works to date. The £5m horse heads will be hard to miss once their construction is completed in September, as the centrepiece for the regeneration project between Falkirk and Grangemouth. The huge equine monuments are not only steel tributes to the beasts themselves, Scott says, but a kind of elegy to the lost industries of the Falkirk area and of Scotland. The Kelpies will take 75 days each to construct from their many pre-fabricated pieces on the Helix site, like "putting together a huge Airfix kit".

"They started building them last Monday, it is unbelievable – at last," Scott, whose hefty forearms and shoulders say much about the physically demanding aspects of his work, says as we sit in his modest office over tea and shortbread. He has temporary red weals on his face from his close-fitting mask. "It was January 18, 2006 when I first had a meeting about that project. The foundations were poured a few weeks ago, and the guys from SH Structures have started to put them up. I can't quite believe it, it's very exciting. The frames will go up first, then the skin cladding, but by the end of September they will be finished sculptures.

"These guys put up football stadiums, so for them it's a reasonably straightforward project but it is the end of a very complex and protracted engineering contract. It has been a masterpiece of logistics from them."

Scott, who graduated from the GSA in 1986 with a BA Hons in Fine Art Sculpture, followed by a Diploma in 1987, has been working with galvanised steel, fibreglass and cast bronze for years but the Kelpies have taken up much time and effort. "To think I first drew them up on a piece of paper on my then-girlfriend and now wife's kitchen table in Amsterdam [Scott is married to architect Hanneke Scott-vanWel] to see those little line drawings go from that to what we are building now is an amazing thing.

"It doesn't happen to many people. I am still a wee boy at heart: and yes, it is pretty cool. Scotland is a small country and I don't know there will be many more opportunities like this – but it's not as if I am hanging up my gloves and retiring. I am keeping going, I have to."

I mention the (smaller at 66ft tall) Angel of the North, the huge sculpture designed by Antony Gormley in Gateshead, Tyne and Wear, a similarly large work which has come to represent the north-east of England and its attempts to rise above the effects of post-industrial decline, and wonder if the Kelpies could be similarly embraced in Scotland.

Scott says: "The Angel captured something, it was there at the right time and the right place, and it did a fantastic job in raising the awareness of how public sculpture can be a catalyst in regeneration, and now, after a decade of that kind of work being done, you begin to hear the chattering classes saying 'there is too much of this'. In the beginning, of course, it wasn't embraced and that is something I have experienced to some degree – the piece at Cumbernauld [Arria, a 32ft, seven- tonne female figure]; it is fair to say there was a mixed reception, the local papers made a story out of it, but now I think it has been well received."

The sculptor is a "proud Scot", born in Springburn, who moved to the south side of the city when he was young. His father Drew worked as a draughtsman and encouraged his artistic inclinations by telling his son to "look up" in the city centre, to admire and respect the work and skill that went into creating Glasgow's grand buildings and statuary. "There is stuff in the city centre which is phenomenal," he says.

As a youth, Scott spent two summers at Castle Toward, once used as an outdoor education centre, a pivotal moment in his life. "Holy moly, I think there is something in this [art], that was what I thought after that." At art school he was seduced by the physicality of sculpture.

Right now, he is busy. The horse he was working on as we arrived is part of a series of equine works commissioned by a private collector in New York. On the wall of his office are the titles and details of other projects. Scott's work depends on commissions and public or private backers, not on galleries. Indeed, he has never had a gallery show.

He says it can be a precarious existence. "It is an interesting field to be involved in, that's for sure. I have never had a gallery show, although I have had open air exhibitions. The enclosed white box gallery has never appealed to me, I am working from one commission to the next to the next."

Scott's career blossomed, albeit slowly, after the heavy horse sculpture alongside the M8 at Easterhouse, commissioned in 1997, which is seen by around 100,000 motorway users every day. "I was working at the time with a commercial company and it was going quite well, but after the heavy horse I thought: this is different. But it's a tough world, and the immediate lack of more work was a real wake-up call.Gradually they (the commissions) started to come in, and one inquiry leads to another. You are still riding by the seat of your pants and I don't know what is happening after this work out there in the studio but I never have. Luckily I have a very considerate bank manager."

Working in steel is hard. It's an unforgiving material and "I have injured myself once of twice, which is quite worrying", he notes, "but you cannot let the material rule you." Even moving the pieces around his workplace requires testing logistics. He works with an engineer to make sure the sculptures work physically. Mistakes are not erased with turpentine, but with an angle grinder and oxy-acetylene.

Of course, his work is not universally praised. Criticism of his style and type of public art does not sit easy with him, although he admits he has learned to live with it better than when he was younger. "I will be diplomatic," he says. "When I was young I used to get myself all bent out of shape, as you do when you are young and daft. I am more at peace with things now.

"People are unaware of the amount of compromise that goes into making a work for a particular client in a particular place. It is easy when it is there to think 'well what the hell is that all about'. I am probably slightly too sensitive to criticism because they don't know what I was asked to do and they don't know what I had to do to get the funding to make the job happen."

His work is in a sphere, then, where he is not entirely free. Scott says: "Some clients are better than others and give you reasonably free rein, others have a very strong idea of why they are commissioning it, who their audience is and what it's for.

"But," he adds, "if I ever was offered a gallery show, I wouldn't be short of things to do: my head is bursting with ideas."