Set high on the rolling, windswept Berkshire Downs, Ashdown House is one of the loneliest and most romantic of country houses. Its tall, narrow proportions give it an air of unreality — it looks like an oversized doll’s house plucked from the playroom and abandoned in the English countryside.

It is a wonderful deception. The honey- and cream-coloured house may be imposing, even dour, from the outside, but inside, despite more than 8,000 sq ft of living space, it is surprisingly cosy and eminently liveable. The scent of lilies hangs in the air; the heavily furnished rooms drip with art and ornaments, many in the same Dutch style as the property itself, which was built in the 1660s.

Even in the current depressed climate, such a jewel of a house could sell for £12m-£15m — yet it is on the market for £4.5m. The reason? It is owned by the National Trust, and that £4.5m buys you only a 41-year lease, for which you will pay a peppercorn rent of 5p a year — although the trust has indicated that it would be prepared to extend it to 99 years for a further £1m.

“It is quite rare for the National Trust to sell long leases on its larger mansion properties,” says Richard Henderson, the NT’s property manager in Oxfordshire. “It is one of the most beautiful properties we have.”

On the day I visit, I am welcomed on the front steps by Maurice, the butler; dressed in a navy jumper and trousers, he is ramrod straight, his face impassive under a huge umbrella. The lessees, I am told, are not at home. A wealthy couple who guard their privacy fiercely, they have allowed me in only on condition that they are not named.

This house is all about civilised indulgence and enjoyment of the finer things in life, aided by the consummate skills of Maurice and his wife. (Both worked at Castle Howard, in North Yorkshire.) He tends to the paintwork: “I match the colour — not the original, but original with fade.”

The lessees, who have several homes across the world, took on Ashdown House in 1984 and have used it primarily as a weekend home. Six years later, they bought the property, which has two lodges, three cottages and 100 acres, on a 60-year lease — the remainder of which they are now selling.

They have lavished millions of pounds on renovations. The centrepiece of Ashdown is an almost indecently huge wooden staircase, its bulbous turned-oak balusters and plinth-like newels polished to within a slippery inch of their lives. Downstairs, the three reception rooms are perfect for entertaining; the interlinking sitting and drawing rooms have been immaculately restored. Below stairs is something of a period piece: the kitchen is an off-white homage to the 1980s, with rustic brown tiles and a dumb waiter that links it to the wood-panelled dining room above. On the counter nearby stands an empty bottle of champagne.

There are six main bedrooms — neat and square, with discreet ensuites — off the first- and second-floor landings, with two more in the attic together with a sitting room. Walk up the seventh and final flight of stairs to the cupola, and from the balustraded platform on the roof you can see for miles. The view is lush and tranquil in the summer; eerie in the low mists and bare-branched winter. Swindon is a 20-minute drive away, but the only sound up here is the cawing of rooks.

There is not another house in sight. The gardens are laid out in a neat parterre, with formal avenues framing vistas of rolling hills in all directions. The manicured parkland quickly gives way to grassy fields dotted with sheep and a wandering herd of deer. Further out towards the horizon are darker patches of trees — huge chunks of which are heaped in baskets in the sitting room, ready for one of the many fireplaces. The eye for detail is in evidence even here: the kindling is tied with glittering ribbon.

Included in the sale is an orangery, designed and built in classical style by the architect Philip Jebb in 1991, on the site of some derelict stables. Covering 1,500 sq ft, it is set in a neatly fenced 14-acre site, a couple of hundred yards from the main house, and is freehold, rather than being on a lease. A buyer would be free to sell it, which could bring in about £1.5m. It is the perfect place to lounge around, or to fling open the triple-sash windows and dive into the 18-metre pool, play tennis or relax in the marble steam room.

One can only imagine what the original owner, William, first Earl of Craven, would have made of such expensive frivolities. Legend has it that he built Ashdown in honour of the unrequited love of his life, Charles I’s sister Elizabeth. Known as the “Winter Queen”, she married Frederick V, the Elector Palatine, and reigned as queen of Bohemia for a year until her husband’s humiliating defeat in 1620 by the Habsburg emperor Ferdinand II, whereupon she fled to the Hague. Craven, a staunch royalist, came to her rescue. Following the Restoration, he lent her his house in London.

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