He'd be the last person to say it, but Andrew Fairlie is something of a style icon.

At 49 he remains boyishly youthful, but his cool credentials go way deeper than his designer chef's whites or his personal style choices of Armani and Dolce & Gabbana.

It's 30 years since the talented Perthshire lad won the first Roux Scholarship, an accolade that thrust him into the world's top kitchens then on to the culinary stage in his own right. His cooking attracted a second Michelin star for his restaurant at the Gleneagles Hotel in 2006, and last year he joined the international culinary elite as one of only 160 Grands Chefs Du Monde.

In the quiet, velvety space of his restaurant, Scotland's top chef, who will be honoured at the Scottish Style Awards this month, looks like a man without a care.

But in fact he's feeling hellish. He's recently completed a second course of chemotherapy in a fruitless attempt to stop the seizures he's been experiencing following a partially successful operation to remove a brain tumour in 2005.

He'd been fine for five years but when he started to suffer epileptic seizures consultants advised chemotherapy to control them. The first programme began in 2011 (which he interrupted in order to climb Mount Kilimanjaro, resuming treatment last November). His second course took place last month. However, it hasn't been entirely effective: he can be fine for weeks, then the seizures can arrive at the rate of 20 a day. So in a bid to shrink what remains of the tumour he will undergo a six-week outpatient course of radiotherapy in January, followed by more chemotherapy.

Each course of chemotherapy entails travelling to the Beatson Oncology Centre in Glasgow to be scanned and to receive five days' worth of tablets to take at home. "It makes you feel like shite - nauseous and exhausted," he says.

"The seizures last anything from 10 to 45 seconds and I can be either standing up or lying down. They only affect my left arm and leg." He shows me how his left index and ring fingers are weak, and his left arm "lazy". This is surely dangerous for a chef, who might be using a knife or holding a hot pan. "I'm aware of when it's going to happen. I get an aura of a few seconds' warning so I can put down anything I'm holding."

He imparts this shattering news with astonishing equanimity. I wonder if this is down to religious faith; but though he was brought up Catholic, he is not inclined towards prayer. "If I was going to rely on anything it would be Buddhism but only as a lifestyle, not as a religion, because all religions are too restrictive," he says. "I do believe in karma, that you'll get out of something what you put into it. If you're nice to people you'll get good things back. I believe in the power of positive thinking. If you wish for things to go bad they will."

This calmness is characteristic of the man once described as a "wild child", always getting into trouble at school and refusing to conform. Everything in his restaurant - unique in its class for having no windows - is thought out to the last detail. When others advised him to paint it white to lift the ambience, he chose a deep brown-black by Farrow & Ball because he loves the texture of its paint. He commissioned the ceramicist John Maguire to supply his unique black crockery, and sourced his striking water tumblers - embellished with distinctive red glass swirls - from a small London shop. Everything is bespoke, from the eye-catching chandeliers, subtle downlighting, deep-pile carpet and giant artificial bonsai tree.

"I want people to feel comfortable and cocooned, as if they could be anywhere in the world," he says.

He commissioned Gregor Mathieson (now his business partner) to design the room, because he was the only person who listened to what Fairlie wanted. "He asked me how I'd like the room to feel, then came up with exactly what I'd imagined." Style details have been influenced by the Glasgow artist Archie Forrest, whose still lifes and portraits adorn the walls of his restaurant. "It was Archie who taught me the importance of presentation," he says. "Working with him was brilliant. He'd explain why putting a red apple in the corner of a painting only takes the eye away from the action in the centre, and the same goes for how you construct a dish. I learned a lot through him."

He rarely speaks in the first person, always referring to "we" and name-checking individuals from his carefully chosen team of designers, suppliers and staff, many of whom have been with him since the beginning: his head chef Stevie McLaughlin and general manager Dale Dewsbury followed him to Gleneagles from One Devonshire Gardens, in Glasgow, where he got his first Michelin star in 1996.

For Fairlie, collaboration is vital, yet he does not place his trust in people lightly. He reckons he's a good judge of character. Trust and loyalty are big words in his vocabulary.

Fairlie is also a big family man. He has two daughters from his marriage to ex-wife Ashley: Ilona, 23, a social worker in Edinburgh; and Leah, 17, who lives with him, his partner Kate White, whom he met several years ago when she worked at Gleneagles, and her daughters, Kitty, 11, and Rosie, eight. Their newly completed home in Auchterarder, a designer haven of floor-to-ceiling windows and reclaimed stone, is crammed with pink toys and wigwams, with the occasional glimpse of teenage make-up.

"It's good fun," he says, beaming. "The younger two love having a big sister."

Dominating the joyfully chaotic space is an open-plan kitchen with a large dining table overlooking the garden and the Ochils beyond. It's a far cry from Fairlie's childhood home, a cramped three-bedroomed council house in Letham, Perth, where he lived with his parents Jim and Kay and siblings Philip, Jim, Sharon and Katrina. Then, as now, noisy family mealtimes were a big part of his life. The young Fairlie would help his father, an economics teacher, cook the evening meal after school so it would be ready when his mother came in from work in a local shop. This is how his interest in food was ignited. "Every single day we would sit down and have dinner together and it was great fun. My father was a huge influence on me."

Fairlie wasn't academic. He left Perth Academy at 15 because he was "bored" and wanted to become an apprentice cook at the Station Hotel in Perth where, at 14, he'd landed his first part-time job polishing glasses. He'd realised he wanted to learn more and was trained by head chef Keith Podmore, whom he followed to London. Aged 20 he won the first Roux Scholarship for young chefs, the prize being a three-month placement in Michel Guerard's Les Pres d'Eugenie in Gascony - an experience he says profoundly affected his culinary approach.

After a series of stints in France and London he joined One Devonshire Gardens as head chef in 1994, got his first star there two years later, set up Restaurant Andrew Fairlie at Gleneagles in 2001, and got his first star there the following year and a second in 2006. He has retained it since.

Until now, all the decision-making has been his. But since last year he's been giving McLaughlin a more hands-on role, and the head chef's food is appearing alongside Fairlie's own.

"It's good because it takes a lot of pressure off," he explains. "I'd wake up at 3am worrying about something because if I didn't change it, it would never be changed. Now Stevie's more involved he's happier and I'm happier."

If a third Michelin star were to come along he'd be "over the moon" but not if it put added pressure on him or his team. "I see stress as a positive thing, but only if I'm in control of it."

The key to being a successful chef is having an understanding of how people tick. "Management of professional relationships is a large part of what I do. It's important to keep staff calm and motivated." McLaughlin will compete in the next series of BBC Two's Great British Menu, as part of his plan to increase his profile and help him "come out of the shadows".

He describes how his cooking has evolved. "The food we were doing 12 years ago when we opened was very precise. I had to have form on the plate. For example there was a pork dish that had three elements and they all had to be in proportion in terms of size and form. Also crucial was having a range of different shapes of plates. There are certain dishes I'd never put on a rectangular plate - chicken breast or a whole pigeon, for example. It just wouldn't look right."

The a la carte remains very neat and clean with much thought going into the composition of each dish. But the introduction of a du marche (market) menu allows for more of what he describes as "freefall style" where a dressing of leaves can be dropped onto a dish instead of being artfully placed with a pair of tweezers. for example "The du marche is supplier-driven, flexible, changing, exciting and interesting. We'd never have put confit chicken wings on the a la carte menu, for example."

This has created a new buzz in the kitchen and front-of-house. "Customers are looking for a more casual two-star dining experience these days. A quarter of everything we cook is for the du marche menu."

This "casualisation" at the top end of the market is very on-trend and will be further developed when the restaurant's Victorian walled garden comes into full production. Fairlie has signed a lease for a two-acre privately-owned garden situated 10 miles away from Gleneagles. It will be headed by Jo Campbell, head gardener at Raymond Blanc's Le Manoir Aux Quatr'Saisons in Oxfordshire.

"We're very lucky to get her," says Fairlie. "She's a natural forager, very into plants and unusual flavours such as apple meadowsweet. We're hoping to grow forgotten heritage varieties from the area. I am sure there are many ancient Scottish flavours that haven't yet been rediscovered.

"This has always been the part of our business that has been missing."

Yet he believes that trying to be different from everyone else doesn't work, just as following fads doesn't either. "It has to be a natural, organic process. We went through a silly period when we tried to do what El Bulli and El Cellar De Can Roca were doing [where ingredients are deconstructed and recreated] but it wasn't natural to us. Customers said they didn't come here for that sort of thing. We prefer to let natural ingredients speak for themselves and be creative around them."

Though he doesn't believe there's a particularly Scottish style of cooking, he says the Scottish terroir - the natural environment in which a particular ingredient is produced, including the soil, topography, and climate, and its power to deliver unique flavour to indigenous ingredients - is "massive" and what people come from all over the world to experience.

Despite his growing international profile, Fairlie remains committed to the land of his birth. His father Jim was deputy leader of the SNP from 1981 to 1984. The chef is not a member of the SNP, but he is a nationalist and is on the board of Yes Scotland. When he "came out" as pro-independence, he got emails from people saying that they'd never again eat in his restaurant. Yet last year was the restaurant's best-ever, and it's already on target to beat that, he says.

"Maitland Mackie [of Mackie's] said people would stop eating his crisps if Scotland became independent. But my view is that if something is made in Scotland it has added value. We need to finally throw off the Scottish culinary cringe."

Fairlie has no fears about turning 50 on November 21. "I don't care about it and I think that's because I'm happy. If I felt life had passed me by, or hadn't done the things I wanted to do, it might be different.

"When I think back to when I was young, cooking for people like the Roux brothers, and now being one of the Grands Chefs Du Monde, I pinch myself and think, 'My God, I've fulfilled my ambitions.' I have a family I adore. I'm in a restaurant I absolutely love. There's nothing else I want to do. Except, perhaps, get over my brain tumour.

"I'm lucky in that the type I have responds to chemotherapy. I see people who are a lot worse off than me at the clinic, in wheelchairs, quite obviously terminally ill. I continue to keep fit by cycling as I always have, and I still get drunk. I don't consciously say I can't do something because of my brain tumour unless I'm feeling really bad."

He's optimistic that research into tumours is ongoing. "The chemo I'm on wasn't available when my tumour was discovered. It will develop again and new things will be tried. I'm very positive about my condition. I feel things will take care of themselves."

And that surely is the very epitome of cool. n

Andrew Fairlie will receive the Outstanding Contribution award at the 2013 Scottish Style Awards ceremony next Saturday in Glasgow. Visit scottishstyleawards.co.uk.