As we sat down for a cup of tea and a chat, I had no idea that I was about to witness one of the world's rarest phenomena – Eric Knowles lost for words.

We were at a hotel in the South of England where Knowles, ever-youthful veteran of the Antiques Roadshow, was hosting a weekend of lectures, valuations and memories of Arthur Negus. In the next room, a wedding reception was in full, noisy swing.

Half way through our first cup of tea, two women teetered in from the reception, merry as only a couple of glasses of champagne can make you. One buttonholed Knowles. "Excuse me," she said. "Are you famous?" For once, he was stuck for an answer.

The other woman, broad Cockney, chipped in: "What's your name?" He told her and she shrieked: "That's it! You're the antiques dealer off the telly. Come and meet my brother…" Gamely, he did, doing the rounds of bride, groom and guests like HM Ambassador of Antiques.

Knowles is a safe pair of hands (particularly with priceless porcelain), so he can be guaranteed to put on a good show at the valuation day he'll be running at Harewood House, near Leeds, in the summer. He'll give an illustrated talk on trade links between Europe and the East, take part in valuation sessions and try to restrain the jokes about Yorkshiremen which, as a Lancastrian, come all too easily to him.

More from Eric Knowles later. For the moment, we fast-forward to Harewood, where Melissa Gallimore, the house's curator, is keen to show off the astonishing 18th century wallpaper that's the centrepiece of the current "China at Harewood" themed season.

The season explores Harewood's Chinese links, taking in kites, dragon boats, calligraphy, contemporary art and much porcelain and jade. Plus the Chinese imperial robe which Queen Mary, the present Earl of Harewood's grandmother, converted into a coat for trips to the cinema.

But the season's real draw is the wallpaper. As we stride through the house, noting one stately object after another, Melissa Gallimore outlines a fascinating lost-and-found story.

The wallpaper, dating from about 1770 and designed for a bedroom, was discovered 20 years ago, rolled up in a carpenter's workshop in an estate outbuilding. It had probably been there, unnoticed except by hungry insects, for 150 years. Mould (and the insects) had got to some of it, but its colours hadn't faded and, after its recent restoration, experts have described it as one of the finest examples of antique Chinese wallpaper anywhere in the world. This is not, it needs saying, wallpaper as we know it today – narrow rolls with repetitive printed patterns. Hand-painted in China on 20 sizeable sheets, it shows scenes from the everyday lives of peasants working in the tea, rice, silk and porcelain industries. It adds up to four huge murals of oriental life two-and-half centuries ago. We're not talking B&Q and packets of Polycell here. The murals, dotted with the odd pagoda and backed by misty mountains, are busy beyond belief. Hundreds of vignettes show men chopping down mulberry trees, planting rice, packing tea and steering cargo boats along fast-flowing rivers. Like a willowy Chinese version of a Brueghel canvas, it's overwhelmingly detailed. And also slightly haunting. It could be the stuff of fantasy... by night, as guests slept in the Chippendale four-poster, the peasants would come to life, working away in the darkness before resuming their picturesque poses at dawn. The guests would wake to find little porcelain bowls of tea and rice left as presents on the bedside table.

The murals' greatest charm is their quirkiness. As well as the predictable fishing and pipe-smoking, one man dozes drunkenly off while working; another blows his nose; a third tweaks the ear of what appears to be a dwarf. Monkeys clamber over tea bushes and there are reports of a man pulling down another's trousers, but you don't like to seem prurient by peering too closely.

All the characters are smiling happily (as peasants traditionally do). Even the dogs and the water buffalo smile. It is, as Melissa Gallimore points out, a romanticised vision of Chinese peasant life created for a western market captivated by the distant and supposedly "exotic" East. It's artistic tourism, echoing what Robert Fortune, the 19th century botanist who introduced tea to India, wrote: "There are few sights more pleasing than a Chinese family engaged in gathering the tea leaves, or, indeed, in any of their other agricultural pursuits." The Chinese families were no doubt gratified to be so pleasing as they toiled.

"Chinese visitors have said the wallpaper creates a Europeanised view of China," says Ms Gallimore. "You wouldn't have got anything as busy as this on Chinese silk hangings made for their own use. To them this would have been vulgar because it's so intensely decorated and has so much colour." The wallpaper was commissioned by Edwin Lascelles, an ancestor of the present Earl of Harewood, for the newly-built house. His brother Henry, a ship's captain in the East India Company, possibly brought it home, after a return voyage to China taking up to two years. The Chinese may have called East India Company merchants "fankwei", or "foreign devils", but there was little hint of westerners exploiting orientals, common enough elsewhere. As Eric Knowles points out, the Chinese merchants drove a hard bargain. "They were nobody's fools," he says. "They were the multi-millionaires of their day. You can't teach the Chinese anything about business." This sort of plain-talking, plus a dollop of dapper Northern charm, has made Knowles the Alan Titchmarsh of the antiques world: an accessible guide to a specialist area.

Take his description of Chinoiserie, the European style inspired by the Orient: "It was often fanciful and ludicrous and inventive. The biggest extravagance is Brighton Pavilion, the ultimate expression of Chinoiserie – bizarre and bonkers.

"And there's so much of this stuff around. At every car boot sale I go to, I get offered a Chinese plate. Often with a crack in it. I've become an orphanage for unwanted china. I buy it so people don't feed the
cat off it."

Knowles' great appeal is that he's not posh; he's not cut-glass, double-barrelled or old-school-tie, as many expect the antiques world to be. His more populist talks are likely to be sprinkled with references to Last of the Summer Wine and Allo! Allo! and, given the right age group, Emergency – Ward 10.

He grew up in a council house in Nelson and, as he says: "I don't try to be anything else apart from me; I'm not there to play the token Northerner. And it's very nice when people say they feel they know me."

As a regular on the Antiques Roadshow for 28 years, he's had plenty of time to ponder the programme's popularity. "It appeals to the psyche of the British," he says. "There's something cosy about it. It's a little bit of comfort and it ticks all the right boxes. It's information. It's a game show. It's entertainment. It's a formula."

And, of course, it's money: the tension of valuation, swiftly followed by jubilation or disappointment. "I always say I'm responsible for more people cancelling world tours than taking them," he says. "But I'm an art broker, not a heartbreaker."

And he's a wow at wedding receptions.

Eric Knowles's Valuation Day is at Harewood House (0113 218 1000, www.harewood.org) on July 8.