P art of the pleasure of looking round the grounds of houses in the National Gardens Scheme is to glimpse worlds otherwise beyond your ken.

Of course, not all the gardens are like that. Your next-door neighbour might open his, if he's proud of his green fingers. But it's the great mansion houses behind high stone walls or down mile-long drives that really pull in the crowds, who for an hour or two can wander around at will and get just a taste of what it is to be a lord of the manor or off-duty captain of industry.

Or, in the case of Whatley Manor, near Malmesbury, what it's like to be a guest at a luxurious country house hotel and spa where rooms start at £290 a night and go up to £850 for the grand suite.

The place prides itself on being one of those glorious retreats from real life that's full of feel-good factor and discreet pampering, from the moment you sweep in through the remote-controlled oak gates to your heartbroken exit down 900 metres of immaculately drystone-walled driveway.

It hardly needs saying that the gardens are a vital ingredient in the magic spell – not vast, at about 12 acres, half formal, half natural, but a world of their own, as all the best grounds should be.

Justifiably, they're the pride and joy of head gardener Barry Holman, who came to Whatley at the start of its transformation, in January 2001, saw them safely through the hotel opening in the summer of 2003 and, with his small team, has been honing and enhancing them ever since. Forty-five-year-old Barry was born in Brighton but grew up in Wisley, Surrey. The fact that the village is home to the Royal Horticultural Society's flagship gardens is not coincidental, since his father was supervisor of the floral department there.

The young boy worked there in the school holidays from the age of 13, and at 16 joined the staff, staying for five years before spreading his wings. His first venture was a landscape and garden maintenance company in Cornwall, but he realised early on that what he really wanted to do was bring new life to large gardens in distress.

The first of these was back in Surrey, the 13-acre Coverwood estate at Peaslake. He had hardly unpacked in 1987 when the famous hurricane Michael Fish never foresaw uprooted 4,000 of its trees, and part of his work there was to plant an arboretum.

He came to this part of the world in 1993, as head gardener at Lyegrove in Badminton, where the grounds had been laid out a century ago by Lady Westmorland, a confidante of the then Duke of Beaufort. By the time Barry reached them they were in a sorry state, but the challenge they presented was the perfect preparation for Whatley Manor.

It was just as well, since there was everything to do at his new post – and, at first, he had some very strange conditions to contend with.

"I arrived in January 2001, and planning permission for the hotel didn't come through until the October," Barry recalls. "Because the building is Grade II listed, and the gardens are part of its curtilage, we weren't able to touch them all through that first summer – though we could work down by the river Avon, clearing brambles and elderberries.

"Then, when construction did begin, the grounds were declared a building site, and we had to work in high-vis jackets and helmets. One thing we did learn about builders was that their safety-consciousness did not extend to any plants that happened to be in their way – but we had to be realistic. We knew their work was the priority. Without the hotel doing business, there wasn't much point having a garden."

The gardens at Whatley Manor were developed in the Twenties by a Canadian insurance magnate, and happily, someone lay their hands on the original plan. A landscape gardener had paved the way for the new work.

"It was a start," says Barry. "The garden had gone through all kinds of changes since then, and I think in the previous 20 years, it hadn't involved more than a chap mowing the grass. Early on, one of the gardeners said: 'Have you noticed? There's no birdsong'. That was hardly surprising, as there was nowhere for birds to live."

All that's changed completely, with boxes for birds and bats everywhere, as well as a vast variety of natural habitat and food. In fact, when it comes to the vegetable garden, a few dozen fewer visiting pigeons and pheasant would not come amiss.

Back in the Twenties, there were 14 outdoor staff, and the new owner, the Swiss three-day event rider Christian Landolt, didn't quite have that number in mind. On the other hand, he and his management team were prepared to entrust a great deal of decision-making to their new head gardener. "The first brief was very clear and very frightening," Barry laughs. "They wanted a garden with 26 separate areas, secluded, quiet and mature by the time the first guests arrived, and giving them lots of scope for privacy."

Like everyone else, the gardeners are sworn to secrecy about the people they bump into in their work. The gossip columns say Cameron Diaz and the Beckhams have been guests, but that's not true. On the other hand, general manager Peter Egli congratulates Kate Winslet on her Oscar in the hotel's current newsletter...

"They told me I had two years to do it – and there was no set budget, as long as they got value for money," Barry continues. "How many gardeners did I want, for instance? I could have as many as I needed. I said I just wanted a few good ones, and that's what I've got – my senior gardener Lesley Yeo, our part-timers Caroline Weaver and Matt Phillips, and the hotel florist Rachel Wilson."

The garden is not 100 per cent organic, but spraying is kept to a minimum.

The hotel talks of 26 different "rooms", but the surrounding hedging is not and does not pretend to be anything like the towering holly, beech, hornbeam and yew "walls" at Hidcote Manor, that pioneering and world-famous roomed garden at the other end of the Cotswolds. At Whatley it's mainly box, and Barry is proud of how that came about: "We wanted to get started right away, but obviously couldn't plant in situ, with all the building work going on. So we bought hundreds of box plants, the cheapest we could get, and planted them in the cheapest window boxes we could find. When it came to planting them out two years later, we had half a mile of instant box hedges – and piles of window boxes to take to market and sell."

Many visitors to next month's open day are likely to be drawn to the long herbaceous and hot gardens, where contrasting colours and scents crowd in on the senses. The loggia garden, with a characteristic leaf and stem water sculpture by Simon Allison, is another memorable corner, and the rose, knot and kitchen gardens are also of special appeal, as long as you are not expecting anything vast. When it comes to feats of landscaping, however, the lower terrace, meadow and riverbank walk are a triumph – set off by meadows and hedgerows on the other side of the Avon, which have nothing to do with the hotel, but give guests a precise snapshot of a Wiltshire rural scene unchanged for generations, without a trace of human habitation.

From day one, Barry was left in no doubt about his bosses' expectations. "They asked me about my ambitions for Whatley Manor, and I said I wanted ours to be the best hotel garden in the South West," he says. "To that, they replied: 'How about the best in the country?'" And while he admits that he might not be quite there yet, he's working on it.