'WE ARE just passing the house owned by 'Glass Bottom' Carrington,' announced the tour party's guide over the in-coach public address system. 'He was the first Lord Carrington and was known universally as 'Glass Bottom' because he had an obsession with a certain part of his anatomy. So delicate did he think it was that he would only stand or lie down, never sit. He is an ancestor of our present Lord Carrington, the utterly useless former foreign secretary who now spends his time going around Europe failing to prevent wars.'

Across Britain at the height of this summer season there are hundreds of coaches full of foreign tourists, off to rubberneck relics of the country's past and learn titbits of our history. But the group cruising through middle England last Friday hearing about 'Glass Bottom' Carrington was no ordinary coach party, ogling ordinary relics: 'Those Mesopotamian incrustations on the roof coving there? Are they in terracotta?' one of them was to ask later in the day.

The Victorian Society Summer School is one of the country's finest, and least known, tourist institutions. Every year 40 or so people (mainly Americans) pay upwards of pounds 1,200 to spend three weeks immersing themselves in Victoriana. As always, this year's contingent was an eminent bunch - including historians from Los Angeles, architects from Chicago and the chief planning officer of Haringey council, north London. And as always their guide was the architectural historian Gavin Stamp, a man of such fogeyish eccentricity that if he didn't exist, Stephen Fry would have to invent him.

'We are about to arrive at Tyringham Hall,' Stamp continued into the coach mike. 'Designed by Sir John Soane. It's a health club these days. They've very kindly offered us all a glass of fresh orange juice,' he added in a tone that made it sound as if they had offered cyanide, 'a beverage you Americans probably appreciate.'

Stamp took over the running of the summer school in 1976, three years after it started. 'I inherited something I didn't like at all,' he explained. 'All lectures and sitting around in classrooms discussing things. These Americans were just not seeing enough stuff. Frankly I couldn't see the point in them coming here at all if they weren't going to get out.'

So he instituted a punishing schedule and began to take his charges not to Tower Bridge, Buckingham Palace and Leicester Square, but to grittier locations, places less indelibly marked on the tourist map. Last Friday they were off for the weekend to Birmingham, Liverpool and Manchester, cities chock-full of Victorian architectural gems.

'Liverpool I feel like I kinda know, because of the Beatles, you know,' a preppily dressed New Yorker explained in anticipation. 'But Birmingham? I have no take on this Birmingham at all. Is it a big place?'

'I'M GLAD to say I'm handing over responsibility to someone else to guide you through Birmingham,' said Gavin Stamp as the coach began to negotiate the byzantine confusion of the city's inner-ring motorways.

There were ironic cheers from the back of the coach.

'Well quite frankly I wish I could hand over responsibility for the entire three weeks,' he retorted. A round of applause rang out through the vehicle.

But whatever the joshing, members of the school clearly appreciated their guide.

'He knows so much. And he's hilarious, great fun. Look at him, I mean, he's so odd,' said one American. Six foot three, with tweed jacket, tie pin sagging to a limp angle and an expression that veered between surprised schoolboy and outraged maiden aunt, Stamp made an extraordinary visual contrast with his sharply dressed, cosmopolitan charges.

'He's remarkable, full of, erm, get up and go,' smiled another member of the party, an architect from Sydney. 'But you know us Australians, always full of tact.'

The thing about Gavin, everyone agreed, was that he could get them into places they would never see as ordinary tourists. In the first few days of the school they had been round some of the greatest jewels of Victorian architecture: the Palace of Westminster, the newly restored Foreign Office and Abbey Mills sewage works in Newham, east London.

'That was my favourite,' said a historian from the University of Milan. 'You know, considering it was a sewage works. It was so very different from what we are used to in Italy with our sewage works.'

And it was not just the grand public buildings Stamp took them to. He has a way with country parsons, persuading them to open up their otherwise locked churches in exchange for a discreet pounds 20 dropped in the collection plate. The owners of private houses, too, let the summer school visit every year. 'I keep them sweet with large bottles of gin,' he said.

The only problem with private houses, however, is that the owners often feel obliged to provide impromptu lectures. On the way to Birmingham, for instance, the party stopped at one house which, Stamp announced, belonged to 'a pompous little man who will no doubt insist on lecturing you'.

And indeed the man did suggest that he say a few words. Unaware of Stamp's earlier introduction, he could not work out why his lofty pronouncements set his audience tittering.

THE FIRST thing that struck some of the Americans about Birmingham was a road sign that read: 'Humped Zebra Crossing'.

'I wonder who'd been humping them before they started crossing,' giggled an interior designer from Boston as he snapped away with his zoom lens.

'Oh for God's sake,' sighed Stamp.

None of the Americans, however, seemed surprised by the way in which Birmingham, which boasted it was the 'Best Governed City in the Empire', had been savaged by the urban motorway builders of the Seventies. The magnificent civic square, built by the Victorians, now has as its fourth side a three- lane highway. Robyn, a designer from New York, said she had seen that sort of urban renewal many times before.

'I've not been to Birmingham,' she said. 'But it seems very familiar to me. This is what we do to our cities, this could be anywhere in America.'

But if the party was unfazed by Birmingham, Birmingham was slack- mouthed in surprise at the party. You expect to see guided walks round Oxford, or Jack the Ripper's Whitechapel; Birmingham's magistrates' court, though, is not usually on the agenda. As the 40 members of the group gathered around to discuss the upbeat design imagery of the court buildings, local lads in trainers and tattoos, waiting for their hearings, stared in astonishment.

And out on the streets, as the visitors stood smiling at architectural puns, or enjoying those Mesopotamian incrustations on the roof of a bank, the gobs, slobs and yobs of city- centre life shuffled past, trying to be funny. Passers-by would stop and look at Stamp as he chivvied his party like a Victorian schoolmaster.

'Something I pride myself on is strict time-keeping,' he said, standing outside the City Art Gallery, drawing an alarm clock from the pocket of his tweed jacket and consulting it with the look of a Swiss railway official. 'If you're not on the bus, you get left behind. One year I left a girl behind when we went to visit Alton Towers. Mind you, she was so unpopular the rest of them encouraged me to go without her. She caught up after a very expensive taxi ride, in Stafford. She was furious. I left my wife behind once.'

After their first week trailing in Stamp's wake, some of the party were beginning to wilt. 'What's Gavin on?' asked an American in high-heeled shoes. 'And whatever it is, can someone take it away from him?'

'Some of them think it's going to be all dinners in smart country restaurants and trips to the theatre,' said Marta Galicki, Stamp's number two. 'They are in for a nasty shock.'

Marta looks after the party's domestic and catering arrangements. She was recruited by Stamp, who found he had no flair for that sort of thing. 'Had to get Marta in,' he explained. 'I had no time for vegetarians and other cranks.'

So, at the end of an exhausting day trudging around Birmingham, there was no leisurely dinner in a smart restaurant. Stamp took them all to the Waterloo pub in Smethwick to hear a lecture on Victorian drinking haunts.

The Waterloo is not in the most salubrious part of Birmingham. A pub just down the road seemed to be the haunt of drug dealers, spilling out on to the pavement with their mobile phones and fancy shirts. Robyn, from New York, felt at home here.

'It kinda reminds me of the Oyster Bar at Grand Central Station,' she said, admiring the Waterloo's tiled walls. The tiles were not the only thing the Americans liked about the pub: trips were organised in and out of the gents to photograph the elaborate Adamant urinals.

When the lecture had finished, a black youth with hieroglyphics sculpted into his hair, who had slipped in to watch from the bar, applauded louder than anyone.

'Have we enough time for questions, Gavin?' Robert Thorne, the lecturer asked.

'Not really,' said Stamp, looking at his clock. 'Oh all right. But just three.'

After two questions, one from the barman, Stamp quickly interjected. 'Right, that's enough, must get a move on.'

(Photographs omitted)