IT goes almost unnoticed as nettles envelop its lower walls and saplings take root amid crumbling stonework almost 250 years old.

Time is rapidly running out for The Ruin, once the largest and most striking of the buildings in the landscaped gardens laid out in the 18th century by William Aislabie in the valley of the river Ure near Grewelthorpe.

For 130 years Hackfall was a mandatory stop on any tour of northern gardens by those with money, carriages and time to spare. In 1772 the artist Turner went there to sketch.

So it's hard to believe that this dilapidated building, existing on a structural knife edge, once literally represented the pinnacle of a visit to Hackfall. Less than a century ago it was considered the most spectacular tea shop in the country as day trippers arrived from Harrogate by charabanc to savour the views across to Sutton Bank.

Since it was abandoned 70 years ago, however, there has been a process of inexorable deterioriation to the vulnerable sandstone and it is feared that the building will not withstand many more assaults from bad weather. Rarely has the word ruin been more appropriate.

It is in this sad context that the Landmark Trust, a national organisation based at Maidenhead, Berkshire, has launched what it has described as an emergency appeal to save The Ruin from obliteration.

In other words, the trust wants as many people as possible, with a bit of vision and a lot of optimism, to cough up hard cash so that the building can be repaired as a holiday retreat for two walkers.

The project is not as outlandish as it sounds, for the trust is putting holidaymakers up in a rich variety of other restored places including the Culloden tower at Richmond, an old hospital near Skipton and a former pig sty outrageously designed in neo-classical fashion at Whitby.

Mr Peter Pearce, director of the Landmark Trust, makes no bones about the condition of The Ruin: "Last year's exceptionally wet weather has literally washed the stone away, leaving the fabric of the building in a critical state.

"In January this year part of the central arch collapsed and in places you can put your hands right through the once solid walls. Without immediate support there is no guarantee that this remarkable building will survive at all."

What makes The Ruin remarkable, according to the trust, is the fact that it is two-faced. William Aislabie designed it on one side as a classical banqueting house and on the other as a dramatic Roman ruin towering over the gardens' natural features of rocky crags and the tumbling river Ure.

The Ruin and other garden buildings were abandoned in 1932, when many large houses and country estates were in decline and Hackfall was sold to a timber merchant.

In 1984 Hackfall was given a grade I listing by English Heritage and three years later, when the area was threatened with commercial development, the Hackfall Trust was set up by concerned locals.

In 1989 the Woodland Trust acquired Hackfall and the Landmark Trust was asked to take on The Ruin, also known as the banqueting house or Mowbray Point, but throughout the following decade difficulties in gaining access delayed conservation work.

The terrace was collapsing and emergency work was needed to stop the building slipping down the ravine. The crisis was only averted by drilling into the rock face to install a retaining wall.

By 1999, after the longest negotiations in its history, the Landmark Trust finally acquired what it regards as a prize in terminal decay. It had overcome obstacles including multiple ownership, death of one owner and planning consent for an architect's proposed scheme. More than 250 plans had to be copied for more than 100 legal documents to put everything into place.

The trust won strong support for an appeal launched when it seemed certain to acquire The Ruin. Getting hold of it cost much more than anticipated and interim work including scaffolding has been needed to prevent total collapse.

The current aim is to augment the funds remaining in the appeal by relaunching it to repair The Ruin as a landmark. No figure has yet been put on the project, but it has been suggested that £750,000 would not be too wide of the mark.

Mr David Sherriff, a York architect involved in the project on behalf of the Landmark Trust, has made it his business to become familiar with the Aislabie family's involvement in Hackfall.

John Aislabie created the water gardens at Studley Royal, near Ripon, and it was his son, William, who designed the complete opposite at Hackfall, described as one of the few surviving gardens demonstrating the wilder, uncultivated face of Georgian taste. Their highlights include a 40ft waterfall, a fountain, a ruined castle and a grotto as well as The Ruin.

If the Woodland Trust's hopes of restoring the main garden features and the Landmark Trust's aim of saving The Ruin eventually come together, both organisations will rejoice.

But Mr Sherriff does not underestimate the magnitude of repairing The Ruin, nor the importance of ensuring that the spirit of William Aislabie's original aim is not lost in the process. One of the first moves will be to clear the overgrown site so that an archaeologist can make a full investigation.

Mr Sherriff, who has had his work on The Ruin delayed by the foot-and-mouth crisis but who hopes to get on site in the spring for at least a year, says: "The trust has told us to get on with it, but the trust needs lots of money and we need foot-and-mouth to clear up.

"The collapse of the central arch made everyone realise that we had to get on with it. There is going to be a great difference here between a conservation job and a normal building job.

"We need to work incredibly fast now, yet at the same time it will be a slow and painful process."

As we approach the partly obscured victim of time across a field beside the Masham to Ripon road he adds: "Hardly anyone knows The Ruin is here apart from walkers and local people."

l Information about The Ruin appeal from the Landmark Trust on 01628 825920 or at Shottesbrooke, Maidenhead SL6 3SW