BBC's latest fly-on-the-wall documentary is surprise hit

John and Ann Armstrong are treating me to a tour of their double-glazing factory on the outskirts of Coventry. Here behind the razor-wired perimeter fence of a grim inustrial unit just off the most celebrated ring road in the West Midlands, it is all smiles. Ann may be known as the "boss from hell" who is happy to lambast her employees as "wankers" before millions of television viewers, but I appear to have melted her icy exterior.

A mix-up means I have missed our previous appointment, inadvertedly igniting the couple's trademark fury. John eventually agrees to the rescheduled meeting, but only if I bring Ann a box of Black Magic chocolates. I've only been able to get Dairy Milk at the railway station, but it seems to have worked. The owners of U-Fit, the third largest double-glazing outfit in Coventry, have taken me to their heart.

The BBC is halfway through broadcasting an eight-part fly-on-the-wall documentary series, starring this most extraordinary couple who are running what - on the face of it - is a most ordinary business. Transmitted every Wednesday night immediately after Sir Alan Sugar's The Apprentice, to whose power-suited corporate wannabes The Armstrongs provides much-needed relief, the show has built a cult following and proved one of the unlikely successes of the season.

So far we have watched in disbelief as the 38-year-old couple launched foul-mouthed tirades against their hopelessly under-achieving sales team. We have sniggered at their Zimbabwean motivational guru Basil Meinie's doomed attempts to turn U-Fit from a bedraggled group of misfits and losers into a team of highly-focused winners. And we laughed uproariously as the Armstrongs, with not a word of French and armed only with an internet translation of their sales brochure, attempted to launch their "conservatoire" business on a bemused France.

So outlandish is the couple's behaviour that some viewers have been left wondering whether the whole thing is a stunt. Other more sensitive souls question if we are laughing with the Armstrongs, or at them. The truth - who exactly this couple are, why they should subject themselves to ordeal by camera and exactly what is their appeal - appears as baffling as one of John's business analogies. Of course part of the joke is in the mind-numbing banality of the product they sell. UPVC is the antithesis of middle-class dinner party sophistication.

John is only too aware of who his detractors are - and he doesn't care. "It's the sustainable hardwood, sash-window cunt brigade," he says as more tubes of polyvinyl chloride are extruded from the factory's main machine. "The people who are down on it have got money, and because they have got money they have got choices, and they think their choices should be everyone else's. But there are tower blocks up and down this country which would be misery for five months of the year without it," he says.

The Armstrongs' politics are as idiosyncratic as their management techniques. According to the show's script, narrated by the lugubrious Bill Nighy, their aim was always to become plastic-window multi-millionaires. And the spoils of a life in double glazing are apparent.

His-and-hers sports cars are parked proudly outside U-Fit headquarters. The couple share a roomy home on the outskirts of town, complete with its own lavish conservatory. Then there is a new villa in the French fishing port of Sète (home to the poet Paul Valéry, John points out) and a new house on the coast of Morocco.

Yet for John it could have all been so different. His political awakening came during the Falklands invasion, when he started writing "socialist poetry", an activity that goes some way towards explaining his view of Tony Blair. "He is a manipulating, conniving little bastard. He's like a Walt Disney toy - the packaging is great, but after 15 minutes you think 'this is wank'."

A part-time job with the Post Office while at school in the mid-1980s brought the young lad from Northampton into contact with militancy and strikes. While his classmates were studying, he was manning a picket line and handing out copies of Socialist Worker. Plans to become a geography teacher came to nothing, although he continues to subscribe to the Gaia hypothesis. "How can spring just happen? It's a massive system but it has taken man no time to fuck it up," he says.

Leaving school with one A-level, John drifted into a series of jobs, from selling espadrilles to hawking Portuguese reprints of popular books. The move into double glazing was to have far-reaching ramifications, not just for his career but also for his personal life.

The Armstrongs met when John, at the time working for another supplier, pitched up in a Coventry office where Ann, then a newly divorced young mother, was working. "I hated school, I was very quiet and shy," she says.

The period following her split from her son Louis's father was a difficult one. "I was working part time in a window company and then one day in walked John," she says, her eyes lighting up at the recollection. The attraction was instant. "We met on Wednesday, had dinner on Friday and moved in together on Sunday," she adds.

Pooling their knowledge and expertise in the industry, they worked long and hard to build U-Fit into the European trademarked company it is today. Turnover is healthy and the company is valued at "a couple of million". Ann puts in less time at the business now, although it is no labour of love. "If he died I'd have it sold before he was buried," she explains, to John's obvious displeasure.

Although financially secure, the last year has been far from easy for the Armstrongs. Midway through the filming, on New Year's Eve, Ann found out she was pregnant. "It was totally unplanned but we were delighted as well as stunned," she says. Like any couple, they began making plans and facing up to the unexpected future with mounting excitement. Then, sixth months into the pregnancy, disaster struck. Ann suffered a miscarriage. The baby was a little boy who they called Isaac.

The pain is still evident in Ann's eyes. She had been baptised aged 30, but since the miscarriage, and another loss within the family, she admits to struggling with her faith. The baby was conceived and miscarried during the filming of the current series, and the producers asked the Armstrongs whether they wanted to edit it out. They decided to keep it in. It was a decision they know they will have to live with and which they also realise will leave them open to criticism when it is broadcast later this month.

"We decided that if people really wanted to see how we lived and ran our business, then we should leave it in. Some people are going to criticise us but we can't just be about the funny stuff," says John, who is planning to build a glass memorial to his late son complete with an example of his poetry. "I don't give a fuck about criticism. You cannot accept something unless you decide to accept it. If you don't accept it then they have still got it."

Ann has clearly enjoyed the publicity the programme has brought. The company has been sent thousands of e-mails, letters and cards, and the couple even get stopped in the street.

There has also been correspondence of a lewd nature. "I get e-mails asking about my knickers," explains Ann. "Others asking when I'm going to leave John. But most of it comes from other businesses. It is easy when you are in a business thinking that the problems you have are unique. They see them here and realise they're not."

It is nearly a year since the cameras left the Armstrongs and the future seems to be offering a bewildering array of opportunities. The business difficulties shown in the series have eased. The sales team has now settled down and targets are being approached, if not necessarily hit. John is developing a new range of illuminated UPVC products, including one he wants to call U-Tit - a Perspex cube in which people can create their own artworks. A U-Fit apprentice scheme, modelled on Sir Alan's, has already drawn 500 applicants.

John has been approached by a publisher, he says, that wants him to write an autobiography. He intends to call it This Plastic Life. There is talk of an anthology of poems with the working title A Slither of Plastic. Ann, meanwhile, looks forward to spending more time in France cooking, and hopes to rekindle her lost passion for photography. But she remains convinced of the couple's essential ordinariness. "We've got no complaints about the series. The thing I find amazing is how they got eight episodes out of us."

For John it is a question of truth, and his JD Salinger-esque distaste for falsehood, including items such as "padded bras, tinted contact lenses or mega-bling packaging". "The Armstrongs is real - it is not the X-Factor or a pack of lies. At worst it is an opportunity to see us both together in a home video," he says.