Grace Dent on TV: Storyville: Hotel Folly – Folie a deux, BBC4

One of the most jarring shows on British television this week didn't involve an escaped killer, fantasy gore, or in fact contain any graphic scenes which the viewer might find disturbing. Hotel Folly – Folie a Deux on BBC4 was a documentary – filmed over eight years, from 2005 – about Helen Heraty and her partner's dream to buy ramshackle 72-room stately home in the centre of York, renovate and turn it into a… well, they didn't want to be hemmed in by labels. It would be a boutique hotel. Maybe, sort of. They hated to use the term. OK, not a boutique hotel, more of a sort of space people could hire. Helen and John met through a broadsheet dating section. After a sudden romance they'd sold up their holiday rentals and homes and paid £1.6m for property. They'd use it for weddings, maybe? Funerals? They'd call it simply Gray's Court. It would be exclusive, not for the hoi polloi, with overnight stays at some point. Hopefully. Once they'd scraped more money together as they were penniless. Helen had seven children, two dogs and a cat that is increasingly sad, thin and hungry as the nightmare, sorry, dream continues.

Folie a Deux – the psychiatric term for "shared psychosis" – would be an equally apt title for the C4 stalwart fantasy home Grand Designs, where couples egg each other on, collude, are complicit in each other's foolhardy approach to risk. But Grand Designs does a very nifty job of keeping their hour of televised shared "madness" jolly. Hotel Folly – Folie a Deux was billed as a frightening look at the British banking crisis and how it left businesswomen like Helen in calamity, but it wasn't really, it was a documentary about human beings like Helen. And not so much human beings like John, as the pair rarely saw each other.

Like most of the couples on Grand Designs or Location, Location, Location, their big dream of a fantasy home life together involved them sleeping hundreds of miles apart. Helen was one of those women who made her mind up and that was that and everyone should fall into line, many of them mumbling "what is the plan?" and quickly realising there is never a cast-iron plan, just the feeling that Helen knows what she's doing and will be tenacious in her approach to it. Out of the madness will come great joy. Hopefully. Maybe she'd open a Gray's Court tea shop. (At this point she couldn't afford tea-bags). They weren't big on the business having restrictive labels. Unsportingly, local bank managers who visited the dusty bohemian palace would have preferred a business plan with clear intentions before handing over a million pounds to get up and running. Or more if Helen could have it. Seventy two rooms, remember, that's a lot of curtains and carpet.

By 2006 the folie à deux was in full, irrevocable grip. The remainder of the documentary – the following six years – was a beautifully layered, highly claustrophobic look at being trapped in Helen's world. Her world, for all its risk and sense of derring-do, was as repetitive as a laboratory rat's and as stressful as an air-traffic controller's. It was one endless carnival of calling banks and cold-calling yet more banks and repeating her circumstances, then inviting bankers and council planners to the house, then rejection, with constant BBC news updates that Alistair Darling had declared the country in its worst financial position for decades, no, a century. Oh and screaming, fighting children. And the rattle of the cat's metallic bowl as it poked it with one paw to signify "I'm hungry". "Where will you all go if you lose the house, Helen?" asked the documentary producer. "Don't know," said Helen. Helen was a statuesque, beautiful woman. Never at any point in this madness did we see her without lipstick, mascara and her game-face fully affixed. In downtime, she had began a wholly unwinnable dispute with the National Trust over the right to use a shared courtyard. Every day for years Helen fought – loudly, verbally, face to face – with van drivers, National Trust members and occasionally the police.

About halfway through the documentary the folie à deux had lost a player. John's architectural company had folded. He was no longer in much of the footage. No matter, Helen would borrow £60,000 on credit cards and open the tea shop.

It was impossible to turn this documentary off, but instead of being pleasurable, it was the closest one gets to feeling slowly televisually strangled. The final scenes saw footage of John in the tea shop in 2010 looking very much like an old man, then a closing scene of the seven children and Helen scattering John's ashes on a beach. He had died of a heart attack. Helen said it was a real shame he'd not got to see the house now it was up and working for events. "We've been doing funerals," she said. "Yes, we're good at, well, funerals." And just as she began to cry, the documentary finally allowed me to stop looking.