Not only does it work, her gift as a dowser is now at the centre of a thriving business. With the recent heatwave, Peter and Sheila Hedges have been in great demand on Devon farms. She locates the path of underground water courses, then he sinks a borehole and installs the pumping equipment.
Their 300-year-old farmhouse sits amid glorious rolling hills on the northern edge of Exmoor. In the spacious garden Sheila demonstrates her skills with her favourite divining rod, a stout twig of hazel. Tensioning the twig between her upturned hands, she paces across the lawn and the end of the stick rises steadily. "There's an underground river running across here," she explains.
Peter, 56, used to work for Esso as a pipeline engineer. Then, 25 years ago, he and Sheila opted for the good life and moved from Sunbury, Surrey, to this remote spot in Brendon, north Devon, to open a B&B.
When they first moved into the farmhouse, any illusions they might have had about idyllic country life were shattered. The house was in disrepair, they had no mains water and the loft was infested with flies. "I nearly left," says Sheila, with feeling.
After a few summers, their existing water supply ran out. Sheila was reduced to washing their three children's clothes and B&B linen in a nearby stream.
A neighbour who had learnt water divining in Africa came to help them locate an old well. "I had a couple of goes and I found the stick turning in my hand," says Sheila, 53. "We were just doing it for fun then."
But they discovered that dowsing was taken very seriously on local farms. "Divining has been the traditional way of finding water for years and years and years," says Peter. "You talk to old farmers and they always had someone who could do it."
Peter became the local plumber and eventually built up business, calling on his engineering experience to become "PL Hedges - The Water Specialist", locating supplies with the help of a dowser from Cornwall. Ten years ago he persuaded Sheila to take over the divining and now their youngest son, Chris, 25, also helps.
They claim Sheila has a success rate of between 95 and 98 per cent, a figure upon which they place tremendous trust. "We guarantee no water, no charge," says the slogan on their van.
"Sheila can walk into a field, look at it, and know approximately where the water is before she's even used the rod," says Peter admiringly. "She can also tell roughly how far down it is by counting how long it takes the stick to come up. She's usually right, plus or minus 10 foot."
Does she have any idea why she is so good at it? She shrugs and laughs. "No. When I first did it I didn't believe in it, but it still worked. It's got nothing to do with the fact that you believe in it. Once you've done it and proved it's worked, your confidence just grows.
"I was even more surprised when I found I could look at a field and know where water was. I just go on site and wander around looking, and there will be an area that really attracts my attention. So I go ahead and divine there and usually that's where the water will be."
They try to find a point where two water courses meet. Peter uses a big tractor-mounted drilling rig to sink a borehole anything up to 250 feet deep. Then a cylindrical electric pump is lowered down to the water supply.
Most of their work comes by word of mouth and not all of it is local. They have struck water as far afield as Haywards Heath Golf Club, West Sussex, and at big country estate gardens in Kent. Five years ago they were called in to find water on Lundy Island in the Bristol Channel. "There was a geologist with us on the boat," recalls Peter. "He bet us there would be no water. He said `I know the island inside-out: there's no water.' It took us a day to find it."
On their farm, they rely on a 150ft borehole at the bottom of the garden. "It's lovely water," he says. "Beautiful water. It's checked under the Environmental Health Act and every year it passes with flying colours. It has iron, but not to nasty degrees, and it's acid water which is what you'd expect in this area. And there's lots of it - we never run short now."
While the Hedges's cup runneth over, in other parts of north Devon villagers have found mains supplies drying up in the exceptionally hot weather. And when the water did flow in King's Nympton, near South Molton, it came out a sludgy brown. There, villagers have threatened to refuse to pay water bills unless the supply improves.
This brings Peter Hedges on to one of his favourite bugbears: why a nation with months of winter rainfall should have hose-pipe bans at the first hint of hot weather.
He blames waste and mismanagement of water. Few would argue in the week after it was revealed that 826 million gallons of water leaks from the nation's pipes every day. Mr Hedges points out a small but significant local example of similar mismanagement. High up on Exmoor he indicates a series of ditches dug along the roadside.
"The National Park Authority has done this with the best of intentions, to stop people driving on to the moors. The trouble is, the moorland is like a big sponge, and by doing this they have cut through the peat which helps retain water.
"So when it rains, you get water pouring out of here and into the ditch. The water causes erosion and runs away down the hill. What they do in the name of conservation could actually cause more damage than what they're trying to prevent."
But have not some also pointed the finger at too many people sinking boreholes?
Peter says: "That's a fallacy. Most of the properties I work on have their own sewerage system. They take their water out of the ground, it goes through the house, washing-machines, toilets, whatever, then comes out into septic tanks. The water is separated out and ends up back in the ground.
"The worst offenders for taking water out of the ground and misusing it are the water companies. They send the water miles and miles to people's homes, then it goes into the sewerage system, miles and miles again, and where do they put it? Out to sea. What a waste!"