The National Trust has concealed recordings of eight celebrities inside benches. Undoubtedly, listening to Claudia Winkleman while contemplating Quarry Bank Mill might help to sensualise the horrors of Industrial Revolution working conditions. And we will one day wonder how we managed to enjoy the 520 acres of Felbrigg Hall without a bench upon which visitors have been invited to "rest their weary bottoms" by Stephen Fry.

To be fair, Winkleman and Fry are among the best television personalities available, turnips in a sea of turds. But, as a National Trust member, the speaking celebrity bench scheme causes me to contemplate the cliche of dumbing down. (As does the Trust's website for Felbrigg Hall, inviting visitors to "look in the library, the 'internet' of the 18th century". Were books only unevolved websites? Why is "internet" in inverted commas? And unless Felbrigg Hall library is full of pornography, hundreds of unattributed Tim Vine one-liners, and thousands of anonymous comedy forum posters saying that I am a "smug ****ing ****", it is not at all like the "internet".)

I joined the National Trust in a spirit of class hatred, and keep my membership card on a shelf next to my CD reissues of the first four Crass albums. I used to be breathless with pleasure at the thought that these massive country piles no longer belonged fully to the bucktoothed scum who inherited them, living in poverty in one wing while Daily Mail readers stamped dog muck and Shippams paste into their carpets. The professional posh man Julian Fellowes last week identified such prejudice as the last acceptable hatred. Hostility he and his oyster-guzzling friends experience would be unacceptable if directed towards the poor. But making jokes at Fellowes's expense is quite different to mocking the disenfranchised.

Fellowes is privileged and well connected. Apparently, he has the ear of the Queen, the hand of Princess Michael's lady-in-waiting, and something unsavoury that once belonged to the Duke of Edinburgh in a pooper-scooper in the glove compartment of his Nissan. Indeed, it is muttered privately in royal circles that Fellowes's obsession with the monarchy has gone too far. I have nothing against Fellowes. I met him when I appeared on his BBC4 grammar quiz show, Never Mind the Full Stops, for money. Like all posh people, he was utterly delightful and entirely incapable of deliberate malice. Why, one could listen to them for hours, going on about what they imagine life is like.

I have mellowed over the years, and now part of what lures me to National Trust properties is not hatred of the posh, but the sadness of these places and their stories, their quiet and dignified tragedy. Fellowes says he believes that the quest for social equality is a pointless folly. Certainly, the cultural and political achievements of the denizens of the Trust's inherited homes, understood through the artefacts they left behind, would seem to reveal them as our natural betters, if only because they had the resources to pursue finer things for their own sake. But who were they really? It seems we can no longer trust the National Trust.

I forget which house I was in when I first saw through the matrix. I was looking at the bookshelves in the lady of the house's recreated 1920s' reading room. Their contents seemed, surprisingly, weighted towards decadent authors, and included a number of first editions of Ronald Firbank, a rather louche figure to find in such surroundings. I asked the guide in the room what sort of person this broadminded reader had been. "Oh," he said, "those books are brought in from a central National Trust depository. It's used to furnish many of the properties. They may not be from this house originally. She may never have read those writers." I felt the whole world wobble. The room had been dressed, like a set. The character of the lady of the house had been implied and constructed by the set-dressers. What was I looking at exactly? What was real? What was imaginary?

I stumbled out to other rooms, to kitchens, into which it was now standard Trust procedure to pump the artificial smell of newly baked bread, to the laundry rooms, where the same is done with artificial odours of fresh washing. Of course, displaying a historic home requires a number of brutal creative decisions to be made – do you maintain the gardens in their 17th-, 18th- or 19th-century state, for example? – but I felt I no longer knew what kind of experience I was supposed to be having. I thought about my own home and wondered if I was real or whether some cosmic National Trust set-dresser had conjured my whole being from a cryptic arrangement of compact discs and comic books.

I was shaken. Although I still visit National Trust properties, I now prefer the country houses where, somehow, the aristocracy have managed to cling on without capitulating, lacking the cynicism to fictionalise their own living spaces. At an ancient abbey on a north Devon peninsula, the perfectly preserved lady of the house passed us in tennis shorts and stopped to chat about the shrubbery, a glorious rare bird, still queen of its own protected woodland. At a great house in Cornwall, ringed by rhododendrons and an ancient hill fort, a volunteer guide showed us the family's collection of golliwog children's books and offered, guilelessly, that they "don't hold with that political correctness down here". The experiences were entirely unmediated. All smells were real, though, admittedly, I remained the source of most of them.

Meanwhile, at the National Trust property, where a woman may or may not have read Ronald Firbank, the smell of soiled undergarments was not recreated in the cupboard below stairs, where the lord had forced himself upon the serving wench. Nor was there blood spattered across the stable wall from where he split fatally the skull of a slovenly groom. I had to imagine that. The National Trust was subliminally directing the way I responded, emotionally, to the raw material of the property, constructing a narrative that it wanted me to follow, to the exclusion of my own interpretation. What was the National Trust? The very name seemed suddenly sinister, the sort of newspeak name you would give an organisation that was neither national nor trustworthy. It seemed like the sort of organisation that would give a bench the voice of Stephen Fry, not trusting its foolish patrons to have their own thoughts while contemplating the hills, the clouds, the future, the past, thinking of things near, and thinking of things far.

The final episode of Stewart Lee's Comedy Vehicle is on BBC2, Wednesday, 11.20pm. A DVD of the series will be available on 20 June. David Mitchell is away