Life's a blast when you work for explosives expert John Kettles. But he's adamant that pyromaniacs need not apply

A tall man in a padded boiler suit opens the back of his Isuzu to reveal an assortment of sacks and cardboard cartons. Methodically he takes out the contents. Red sticks of nitroglycerine; coils of green detonating cord; yellow flakes of TNT like finely grated cheese; British plastic explosive with the texture of white almond paste; the stretchier, doughier American variant; purple-and-white granules of PJ12 like the sugar-coated seeds in an Indian restaurant. A car full of high explosives. We're here - why else? - to blow them up.

If you've seen a firework display over the past seven days, there's a fair chance it was staged by John Kettles and his men in yellow reflective vests. They did 12 around Scotland. But fireworks are just a sideline for Perthshire-based Blast Design. Their core business is the bigger bang.

Past-their-shelf-life high explosives; rocket propelled grenades; Second World War mines littering the sea bed; oil wells; chimneys; tree stumps; fake meteors and other film special effects; a redundant airliner whose charred wreckage was later displayed in a Glasgow art gallery… you name it, Kettles and his crew will blow it up. He also makes a mean IED (Improvised Explosive Device), and is one of the Ministry of Defence's go-to guys for mocking-up terrorist attacks.

Two days after shoe-bomber Richard Reid failed to blow up a plane in 2001, Kettles was explaining just what went wrong to a roomful of personnel from a government agency he doesn't want named in print. After a car-load of propane canisters was driven into Glasgow Airport in 2007, he was commissioned to re-stage the attack in controlled conditions, and make it work. Not a difficult task, apparently. Had the terrorists been less incompetent things would have turned out much less happily. "There's a lot of ignorance out there," he says, deadpan. "It never ceases to amaze me what people try to do."

Kettles, 50, lives a life every wee boy - and quite a few bigger ones - would envy. If you forget the bald head and 6'2" frame, there's a touch of the overgrown schoolboy about him, too. That sudden high-pitched giggle. His habit of referring to MoD scientists as "squeaky beakies". His delight in pointing out the errors of people in authority. Above all, his enthusiasm. He's deadly serious about safety and running a business, but anyone can see he's having a ball.

Once he starts telling stories, you'll be there all day. There was the night straight out of the movies when he collected a cattle-truck full of RPGs from a Bulgarian arms dealer in a leather jacket and dark glasses (it was 5am)… And the weeks he spent working for an oil company in Saudi when he slept in the desert and wore Arab robes to reduce the risk of being shot... And the day, a fortnight before the G8 summit in Auchterarder, when a team of American and British security goons descended on his farmhouse, demanding that he destroy the high explosive in his store. "I said: I tell you what I'll do. I'll put it under my bed, because terrorists never look under your bed'." There was more of this merry banter before the men in earpieces departed, leaving Kettles' explosives intact.

It's not a job for lovers of the nine-to-five, but the men in yellow vests at Blast's sand-quarry test site have no complaints. Five months ago, bearded Brummie Chris, 24, was a sailing instructor at an outdoor centre. Neil, 27, - known as "the Cockney" despite hailing from Northampton - is a joiner originally hired to work on Blast Design's boat. (No surnames for these chaps, Kettles insists, on police advice.) Now they're explosives engineers, members of a fluctuating crew of up to 20 on the payroll. One day they're lighting fireworks; the next, blasting a fish pass into the bed of the River Dee. Or - today's task - destroying unwanted TNT. Brian, 32, is the only one of the three with the military past you might expect. A nurse from St Andrews, he has a background in the reserve forces and emergency planning.

All things being equal, Kettles would rather not hire ex-army personnel. He doesn't trust them to respect his trade secrets: they've got too many mates working for his rivals. Also, they tend to be a bit gung-ho. "The guys who work for us are just steady people who you can trust. Very laid back. They're not pyromaniacs," he says. "Every fortnight we get emails from people: 'here's my CV. I've always wanted to blow something up'." He pauses for effect. "Delete."

And yet, there's no getting away from it: blowing things up is a buzz. The evidence is all around us. In the Cockney giggle after each explosion. In Kettles' hilarity when the fallout from a blast leaves his head coated in sand. In Brian's non-stop jokes. ("Assume our safety positions!" he barks, crouching behind my back.) Even I'm not immune. There's nothing quite like feeling the kick of a pressure wave reach your chest a split-second before you hear the bang from 25 kilos of TNT. Then that strange peace as you watch the plume of black smoke dissipate in the crystalline air.

Of course, explosives have their ugly side, but Kettles insists he wants nothing to do with it. "We don't do any offensive stuff, we only do defensive," he says. They test weapons like those Bulgarian RPGs to help the MoD work out how to defeat them, or mitigate their effects, and they design and market their own range of protective products. "We were asked could we make a better grenade that would kill more people, and we said no."

The career path from boyhood on a berry farm in Longforgan, Perthshire, to counter-terrorism is not an obvious one. For the first 15 years of his life, he assumed he'd be a farmer. Then his father gave up the farm. At 16 he was training for a career in hotel management, but he missed working outdoors. In his twenties he was a forestry instructor, felled trees for Scottish Hydro Electric, and rejoiced in the nickname Danger Mouse. He was in his early thirties when Scottish Hydro pensioned off the men who blasted holes for the poles carrying powerlines. Would Kettles take an explosives course and do the job as a private contractor?

It didn't take him long to realise that there is an astonishing - and lucrative - variety of uses to which explosives can be put. He started a sideline in moving them around. He was chartering so many planes that an instructor at Perth Airport told him he'd be as well learning to fly. He did, bought his own plane, and diversified into aerial photography and parachute displays. Later he found himself seismic testing for oil and, by the same left-field logic, bought his own boat and designed a monitoring system to allow marine biologists to track whales and dolphins. The fireworks were another sidestep, five years ago. Now the award-winning displays designed by Kettles' son David are a significant part of the business.

It was signing a contract with a zinc mine in Kilkenny to fly high explosives into Ireland that first brought him into contact with the spooks. It was only a matter of time before the Ministry of Defence spotted him as a resource. Kettles doesn't just have a flair for blowing things up, he's the sort of lateral thinker who can work out how al-Qaeda might try to blow things up, and how to foil them. That's a skill the British military needs. "The army don't make anything. They don't design anything. We design and make it and give it to them in a foolproof kit."

His conversation is peppered with references to contacts in hush-hush corners of the government and police. Blast runs five-day familiarisation courses to give British anti-insurgency specialists their first hands-on experience of high explosives. But Kettles remains a commercial contractor whose services are available to foreign states and companies. Last month he was off to Iraq and Kenya. The first time he flew to Libya, to do a safety audit, he was approached by a man who handed him a replacement laptop and mobile phone, instructing him not to take his own on the grounds that "you've got so much information on them, if you lose them we're in real trouble".

He has worked in several Middle Eastern countries, training oil industry workers and advising on the storage and destruction of high explosives. He was hired to test the security systems at Doha airport in Qatar and, much to his employers' chagrin, smuggled nitroglycerine past the scanners. Doha invested in equipment to close their security loophole and the skies are safer as a result. Which reminds him of the time he was going on holiday and his wife was stopped by airport security because her handbag was bobbled with plastic explosive from the footwell of his car… And he's off and running with another story.

Did I mention that John Kettles could talk?