Country & Garden: Country Matters: Heard the one about the Pope and the taxi driver's brother?

On certain days a peculiar fragrance steals into the cavernous, empty ground-floor rooms of Woodchester Mansion, the huge country house buried deep in a valley near Stroud. The smell, which is like that of mothballs, is not unpleasant; but the fact that it survives at all is strange, for more than a quarter of a century has passed since the cellars of the house were used as a laboratory for visiting students, and nobody can make out how the smell persists.

This is only one of the strange elements that haunt the mansion. It is true that nobody has seen the headless ghost that is supposed to promenade the valley road, or the two black dogs said to presage an imminent death, but the house has enough secrets of its own without external apparitions, and the further researchers delve into its history, the more curious the story becomes.

We know for certain that the building began to take shape in the mid- 1850s and was abandoned unfinished a dozen years later. We know that the man who commissioned it was William Leigh, a well-to-do philanthropist and Roman Catholic convert. But what was the great Gothic pile for?

Until recently it had seemed that Leigh intended to live in the house himself, and to recreate a medieval community, monastic and self-sufficient: hence its ecclesiastical architecture, its handsome chapel built as an integral part of the main structure, its bakery, brewery, laundry and so on. New research, however, has suggested that he meant the mansion to be a retreat for visiting Catholic dignitaries, or even a refuge for Pope Pius IX, when the Vatican was threatened by Napoleon III in 1862.

Either way, the design of the house is extraordinary - and here again, research has changed ideas about how it evolved. Tests have revealed that the front and back halves are imperfectly aligned and must have been built separately. Leigh commissioned the first sketches from Augustus Pugin - the pioneer of Gothic revival already famous for his design of the new House of Commons - built between 1843 and 1846. But patron and architect fell out, and Pugin's drawings were developed and executed by Benjamin Bucknall, a young architect who had grown up near Woodchester. For years experts believed that Bucknall was responsible for the whole house. Now it is thought that the back half was designed by Charles Hansom (brother of the inventor of the Hansom cab), and only the front part by Bucknall.

This front portion is by far the grander - deep buttresses, steeply pointed arches, carved mantelpieces and handsome door-frames, all beautifully executed in honey-coloured Cotswold stone, more fit for a cathedral than for a private dwelling. The back is plainer, more domestic, though also exceedingly solid. Both halves have extravagant touches: just as nine stone owls lurk along the rear roof-line of the inner courtyard, so water- chutes in the form of grotesque stone gargoyles sprout from the front of the south range. For architects, the fascination of the mansion is that it was never finished, and that many floors and wall-facings were never put in place, so that Victorian building methods stand revealed as nowhere else. Yet it seems that the Pope, or any lesser mortal who inhabited it, would have been atrociously uncomfortable. No provision was made for heating the large, high rooms except open fireplaces; there was apparently only one lavatory, high up a staircase, and only one bathroom, in which the bath was carved from a single block of stone.

In spite of diligent research, nobody has discovered why, after immense expenditure, Leigh pulled out of the project in the mid 1860s. He himself had been living in another substantial house, The Cottage, on the lip of the valley directly above the mansion, and had no need of an even larger dwelling. He may have run out of money, or become depressed because his health was failing. He died in 1873.

His son, known as Squire Leigh, left the shell as it was, but his grandson Vincent furnished part of the north range in some style and lived there during the early years of this century. Later, the building was let to a farmer, who kept cattle in the grand rooms on the ground floor. During the Second World War, the house was used as a store by American and Canadian troops, and in the 1950s a physicist and teacher called Reginald Kelly set up scientific laboratories in the cellars. He also appointed himself unofficial guardian of the property, making heroic efforts to keep the gutters and rainwater pipes clear.

After his death, the house deteriorated fast. In 1987, Stroud District Council bought it for pounds 20,000, and had to spend another pounds 30,000 on emergency repairs to save it from falling down. Today, however, thanks to the vision and tenacity of local volunteers, the mansion is in better shape than it has been for decades.

In February 1988, a few enthusiasts formed a conservation group, and in 1989 this evolved into the Woodchester Mansion Trust, a registered charity that took the house from the council on a 99-year lease at a rent of pounds 1 a year. The aim from the start has been not to complete the building, but to stabilise the structure and maintain it, both as a training centre for stone masons and as a unique exhibit of Victorian architecture.

Volunteers began opening the house to the public on some summer weekends, and their efforts have raised over pounds 20,000 a year towards running costs and repairs. But by far the biggest boost to its conservation came in 1997 in the form of a grant from the National Heritage Lottery Fund, which financed the first major repair - of the grand stone staircase leading up from ground to first floor. This project cost pounds 300,000, and although finance is secure for the next phase of work, on the west range, further substantial sums are needed, not least to salvage the soaring chapel, which at the moment is packed with scaffolding. The overall cost of repairs is estimated at pounds 3m.

From the stone monkeys which sit on pinnacles of the roof, to the greater horseshoe bats which live in the brewery, and the face of the green man (a pagan symbol of renewal) peering down from the ceiling of the chapel, the mansion remains an astonishing creation: as Winston Churchill said of Russia, an enigma wrapped in a mystery.

Further information from The Woodchester Mansion Trust, 1 The Old Town Hall, High Street, Stroud, Glos GL5 1AP (01453 750455)