It is rare that a Grade I-listed garden comes up for sale – after all, there are only 163 of them in England and Wales, as opposed to 6,500 Grade I-listed buildings. It is even rarer still when it is one as splendid as Leonardslee, which covers more than 225 acres of lakes and woodlands in an Area of Outstanding Natural Beauty near Horsham in West Sussex.

More than a century after his family first settled there, Robin Loder, 64, and his children, who now run the gardens, are reluctantly selling up. The £5m guide price includes not just the gardens, but also a relatively modest six-bedroom house built in 1986, four cottages, outbuildings and greenhouses.

What is not included, however, is Leonardslee House, the Grade II-listed Victorian mansion that was the home of the Loders until the mid1980s, when it was converted into offices and the family moved to the new house beside it. The mansion is now the headquarters of Insight, a human resources and management consultancy, and is also – separately – on the market for £3.25m.

The family has been at Leonardslee since 1889, when Sir Edmund Loder, Robin’s great-grandfather, bought the estate from his inlaws, the Hubbards. In 1907, the gardens were opened to the public, showing off, among other things, the huge-headed, scented rhododendrons bred by Sir Edmund, known as the ‘Loderi’ varieties. The original plants are still growing there.

Today, 40,000 to 50,000 visitors a year come to admire the views and flowers. Unlike with buildings, there are no statutory restrictions on a Grade I garden – and registration, as Robin puts it, is “an accolade of quality, as much about the landscape as the planting”.

Apart from a clipped yew by the entrance, which has a tree preservation order, the individual trees and plants are not protected. “It would make a good golf course, wouldn’t it,” he jokes. One can almost hear the thuds as the “tree huggers” (as he calls them) fall to the floor in a dead faint at the idea.

A rockery, created by James Pulham and Son, the famous Edwardian landscaping firm, filled with evergreen Japanese azaleas, sits at the top alongside the mansion. There is also a deer park with sika, fallow and axis deer, introduced by Sir Edmund. Most of the garden is centred on the valley and seven lakes below, however. As well as magnificent tree specimens such as sequoias, Scots pines and cedars, the valley is bursting with flowering shrubs such as magnolias, camellias and azaleas, with the garden coming into its full force in May.

“My favourite time is in the first half of May,” says Robin. “The lakeside azaleas are in flower in the second half, but I find half-open flowers are more exciting than half-over ones.”

After taking over from his father, Giles, and running Leonardslee for 26 years, Robin knows every inch of soil, every branch and tree, and has a battery of bons mots and stories about the garden and its features. These include the 40 or 50 resident wallabies – he’s not sure precisely how many there are, as “they don’t stand in line to be counted” – which supplement the seven gardeners. “We use them as mowing machines: they need no wages, no petrol, no pensions and they never go on strike,” he says. “And mowers don’t keep their replacements in their pockets.”

And then there is the time, in Giles’s day, when the gardens doubled as the Himalayas for the 1947 film Black Narcissus. “My parents told the story about the crew coming back to film and the rhododendrons were over, so they stuck red crepe on the bushes,” he says. “I still can’t see where when I watch the film.” The hoses of Horsham fire brigade were also brought in to provide a shower over Deborah Kerr’s head.

Three years ago, Robin and his wife, Jane, 60, moved to Netherfield, an hour away, handing over the everyday management to their twin children, Tom and Mary, 36. Robin, though, is not quite able to keep away and is back at Leonardslee about three times a week. Mary deals with back-office matters such as the shop and restaurant, and Tom is in charge of the garden.

“My father was running around doing everything, so I took over running around doing everything,” says Tom, who has a background in information technology. It was a “steep learning curve” – he hasn’t inherited his father’s love of plants, unlike his elder brother, Christopher, who runs a nursery nearby.

After three years of working flat out, Tom has realised he doesn’t have the passion needed to keep the place running as a profitable business. Mary, who has three children, also has her own farm to look after. Leonardslee is glorious in late spring and, if the weather has been favourable, in autumn when the leaves turn. For much of the rest of the year, however, it can appear a little disappointing to visitors expecting floral displays and colour.

The family has tried to add value by planting summer-flowering hydrangeas and holding a contemporary sculpture exhibition in the courtyard, as well as displaying a collection of vintage cars and an exhibition of a miniature estate, called Beyond the Doll’s House. Yet there is something old-fashioned about the place, not only in the terms of the planting (blowsy woodland blooms in pastel loo-roll shades are far from trendy perennials and grasses), but also in its failure to provide an all-singing, all-dancing Alton Towers-like visitor experience.

“The industry has become a lot more competitive,” says Tom. “My aims have always been in another direction, and I can’t really keep both going at the same time. Without a real passion for the place at its centre, it won’t run as well.”

Tom and his young family live in the house built by his parents when they sold off the main mansion. It is no beauty and one wing is used for conferences. Situated at the top of the valley, with views down to the lakes, it would make a stunning location for a new house. They plan to move to Lewes or Brighton, and he will resume his career in IT. There are similar views from the mansion, which has retained some of its original Victorian features, such as a double-height atrium, a large and ornate dining room, decorative fireplaces and furniture designed for the house.

The mansion’s price tag is optimistic – the market for country-house offices is not buoyant at the moment, although the parking for 44 cars and garden views would make it ideal for a wedding business or the like. Richard Purchase, Insight’s chief executive, admits he is taking advantage of the garden’s sale. “We are being purely opportunistic,” he says. “We don’t have to sell. If someone makes us an offer, then great, but we are very happy there.”

More obvious would be to buy both the mansion and the grounds. The estate, thus reconstituted, could appeal to a wealthy garden-lover or perhaps to a Russian oligarch or other foreign buyer looking for a home with large grounds within easy reach of London.

It is not for someone looking for a quick profit. “I don’t think there is a huge marriage value at the quoted price for the mansion,” says Richard Gayner, head of the country house department of Savills, which is selling the garden (but not the mansion). He estimates the two together would sell for £9m-£10m.

Robin is putting a brave face on the sale – and the break it will mean with more than a century of family history. “Jane and I are not getting any younger and it has been a family decision,” he says. “We’ve had a lot of fun and it has been a challenge and a privilege to build it up. It’s much harder work for the next generation to follow.”

The fate of the estate, its plants and the business will depend on the new buyer. But there are at least 40 small furry members of staff included in the sale: with potential replacements in their mothers’ pockets.

Leonardslee Gardens are for sale for £5m with Savills (020 7409 9905 , www.savills.co.uk). Leonardslee House is for sale for £3.25m with Stiles Harold Williams (01293 441305, www.shw.co.uk)

Making the Grade

There are 126 Grade I gardens in England, and 37 in Wales (Scotland has a different system). To attain the status, a garden should be considered to be of exceptional historic interest on the basis of its features, ornaments and overall layout. Unlike listed buildings, Grade I gardens do not enjoy statutory protection – which means, in theory, an owner could chop down all the trees and uproot all the plants, unless they are specifically protected. Local planners are obliged, however, to consult English Heritage before granting permission for projects such as a new greenhouse or a house. If you have chopped down the all trees that made the garden special, they may not look too kindly on your application.

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