A forceful character, she had grown up at Longleat House in Wiltshire and by her standards Harewood was cramped. Also, although cutting edge when it built between 1759 and 1771, its interior was dated. So in the 1840s she became the driving force behind its remodelling.

There was an extension that helped accommodate her 13 children and she replaced the original Adam dining room and some of the furniture and furnishings.

Fortunately for history, grand piles like Harewood, near Leeds, have lots of storage space where unfashionable items can be stuffed.

But while most have been carefully catalogued, first by stewards and now by curators, they are still capable of yielding some surprises.

The latest hidden treasure to be unveiled is thought to have been another casualty of Lady Louisa's makeover.

The precious hand-painted Chinese wallpaper was discovered tucked away in the rafters of a carpenter's workshop on the estate, where it had survived unnoticed for 160 years.

The remarkable find was first made 20 years ago and it has taken until now to restore and rehang it.

The 240-year-old wallpaper is unlike the pre-printed rolls of today. It was made from mulberry paper glued onto lining paper, then onto canvas and stretched onto batons. This enabled the 20 wall-mounted sheets to be taken down in tact.

It is a fabulous, mesmerising hand-painted work of art with colours made from natural pigments such as azurite and malachite

Analysis by Allyson McDermott, one of the country's foremost wallpaper restorers, reveals that it dates from the 1760s and is one of the finest examples of Chinese wallpaper anywhere in the world.

Research shows it was commissioned by Edwin Lascelles for the newly built Harewood. His brother Henry, captain of the East India Company, possibly acquired it.

The paper, almost certainly from southern China, was carefully stored in the west attic at Harewood until recently when enough funding was found for a project to restore it.

Although it was unfaded, it was dirty and had been attacked by mould and eaten by insects.

It was dry cleaned using a tiny vacuum, soft sable brushes and conservation sponges. The acidic backings were replaced with Japanese tissue paper.

Ms McDermott said: "The amazing thing about this paper is that it is unfaded, because it was hung in a room that wasn't very light. A lot of the original jewel-like colours are still there and that's what makes it unique."

The wallpaper reflects everyday life in 18th century China. There are landscapes and pictures detailing the production of rice, tea, silk and ceramics in exceptional detail.

Every character has a smile despite the labour-intensive tasks and there's a lot of humour in it too. One of the men has clearly been drinking too much rice wine. But it is probably a romanticised version of the truth.

"In some ways it's a very accurate description of the industries, but it was stylised and fantasised for the European market who thought of China as this fantastic magical kingdom, " Ms McDermott said.

What she describes as an antique work of art is now on the walls of Harewood's east bedroom surrounded by the four poster bed, clothes press, commode and two night stands that famous furniture maker Thomas Chippendale created especially to go with it back in the 1700's.

The fireplace here is slightly smaller than the room it was made for, so a small part of the paper has been digitally reproduced.

To ensure it is conserved, the temperature and humidity in the room is controlled and ultraviolet filters have been fitted to the windows to protect the paintings from the light.

The East bedroom project cost £125,000, which is probably about a third of the paper's worth. A smaller and more inferior paper sold recently for about £300,000.

The Chinese wallpaper in the East bedroom is now on display at Harewood House