Karl von Wettinberg has so many Christian names that he can't remember them all in order. He has changed his name by deed poll to incorporate a huge glob of text that reads like a page ripped at random out of the Almanach de Gotha: "His Imperial and Royal Highness Prince Karl Fredericke Phillipe Charles Louis Alexander Nicholas Leopold Albert Edward..."

Karl von Wettinberg has so many Christian names that he can't remember them all in order. He has changed his name by deed poll to incorporate a huge glob of text that reads like a page ripped at random out of the Almanach de Gotha: "His Imperial and Royal Highness Prince Karl Fredericke Phillipe Charles Louis Alexander Nicholas Leopold Albert Edward..."

There's not really space to write them all here. He's also persuaded his mother to adopt a similar identity. It's all part of their masterplan to claim the titles and privileges of Charlemagne, and make Teddington, south-west London, the centre of a new Holy Roman Empire. "We're not cultists," protests the emperor's mother. "We are completely sane."

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It's hard to say who or what Karl and Maria were before they decided upon their eccentric scheme of self-aggrandisement. Maria claims she was once a theatre designer, but she doesn't do this kind of work any more. Karl says that he doesn't have a job or a chequebook, and that he has never attended university ("Not that I can't get into them," he asserts.)

They won't reveal their real names, but Karl professes that he has always been Karl, that he is "twenty-five-and-a-half" years old, and that he spent the first three years of his life in South Africa, where his estranged father still resides. Is this man aware that his wife and son are attempting to establish themselves as de jure monarchs of European Christendom? "No," volunteers Maria. "He's very unwell, and he wouldn't like it."

Founded AD 800 by Charlemagne, the Holy Roman Empire expanded from an initial power-base in the German states to encompass territories in Italy, Bohemia and Burgundy. It also claimed sovereignty over lands east of the Balkans, although nobody east of the Balkans ever took much notice. Voltaire's remark that it was "neither holy, nor Roman, nor an empire" was broadly true for most of its thousand-odd year history. In 1648, the Peace of Westphalia removed the emperor's remaining powers, and transformed the empire into a purely symbolic entity. In 1806, it was formally dissolved by Francis II, who feared that Napoleon might grab the title for himself.

The dissolution, however, is not going to put off the Wettinbergs - or whoever they really are. Karl is not asking anyone to believe that royal blood flows through his veins. Maria is not claiming to be some long-lost Habsburg princess smuggled out of Fugger zu Kirchburg und Weissenhorn in a hatbox.

Von Wettinberg is not the name of an ancient family of MittelEuropean nobles - it's simply a word Karl has plucked out of the air. Von Knackwurst would have done just as well. He's quite up-front about being an ordinary bloke from south-west London who's decided that, as nobody has been Holy Roman Emperor since 1806, and as the title was elective rather than hereditary, there's nothing to prevent him from legally establishing himself and his mum as the Royal Electoral House. Nothing but an appreciation of the ludicrous nature of the project, that is.

Last month, they took out an advert in this newspaper, cramming several thousand words of royal proclamation into our Public Notices column. "All and Singular Believers in Christ, Sovereign Powers and Courts of Europe" were informed of Karl's usurpation of the empire. "He's informed the Pope," says Maria, proudly.

"I have a signed document," says the young emperor. "An actual signed document. Of proclamation. People have sworn allegiance to me." And who might they be? "A couple of people. My mother. My sister has. At present that's about it."

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Is life in Teddington so catatonically tedious that a boy and his mother have to resort to fantasies like this? But there's a hard-nosed objective to the Wettingberg project. "It's an opportunity to set up societies, an opportunity for the conference of titles. Pageantry. And I'm not talking about Walt Disney theme parks. Because I'm an amateur historian, I don't want anything like that."

Now, I feel, we're getting to the reason why his mother chose to blow £4,000 on an advert on these pages. Might Karl, perhaps, be in the business of title-selling?

As he's taking his own tape-recording of our interview - a procedure that I've previously known only Michael Winner to bother with - I'll give you his answer absolutely verbatim: "Errr. No, I wouldn't. I can't say. Err." A little later his denial is more firm: "I will never sell any titles or honours whatsoever at all. I'll say that on any record. You will never see an advert saying, 'You too can be Countess Such-and-Such for £10,000.' That will not happen. But..."

"He's got it all worked out," beams his proud mother. "I can say, if you want to give me a gift, I can confer a title." Is there a difference, I wonder? "The difference is, they are not for sale," explains Karl. "If you were a very wealthy man, you could give me a hundred thousand. But if I didn't particularly like you, I wouldn't give you a title."

And what if you really liked me, Karl, what would happen then? "Well, if you wanted a title, then you would give me a stipend. Or a gift. You must realise that you are not buying the title. Say if you were interviewing a really famous person, you would give them a gift. You're not necessarily paying for the interview, you're paying for his time and his patronage."

Well, I once gave Coolio a copy of Golf Monthly, but he never made me Margrave of Meissen in return.

I fear, however, that the Wettinbergs will have little trouble finding people thick enough to surrender their cash in return for the favours of the Teddington branch of the Holy Roman Empire. Title-selling is a minor, but lucrative and legitimate business in this country. Excel Multimedia in St Albans offer to make the punter laird of a "souvenir plot" of Caithness land for only £99. Noble Titles of Mouse Lane, Steyning, Sussex, has nine titles on offer. For ten thousand quid, you can be lord or lady of Kidbrooke (a suburb of south-east London within gobbing distance of Eltham). For £4,500, you can go over to Stubbington, Hampshire, and encourage people to tug their forelock and call you "squire".

"Our company's service," their brochure reassures potential customers, "is based on the fact that the English Legal system allows titles to be claimed and registered, if it can be proved that there are no living heirs... Technically, we are able to register a 'possessory title', which can be sold on for a fee. We make extensive searches of records and archives to ensure that the Titles can be sold, and you can be sure that the Titles are sound." Basically, if you can find a reference in the Domesday Book to a Lord Feckless of Scunthorpe, and nobody holds the position today, you can assume the title and sell it on to some brag-hungry third party.

"If he confers titles," argues Maria von Wettinberg, "it's going to give people who are interested in history the opportunity to help bring this whole thing alive. To become part of something. A lot of good can come from that."

Her son has been nursing these ambitions since he was 15. "I've always been a monarchist, but not in the term of a sycophantic monarchist, you know, kiss feet and bow, bow, bow. I'm a monarchist in its real terms, not in its 'I'll die with you on the battlefield' kind of thing."

Which is a bit of a shame, really. During our interview, over coffee in the Royal Garden Hotel, Kensington - where else? - I've been struck by Karl's wholly unromantic attitude to his emperorship. He's not full of dewy-eyed anecdotes about the glorious victory of Otto I over Berengar II of Italy. He doesn't smart over how, at the Concordat of Worms in 1122, Pope Callistus II cheated Emperor Henry V out of the right of spiritual investiture. It's as if he wants the titles much as a trainspotter might want a complete list of 1950s locomotive serial numbers.

Surprisingly, he's expecting a fight to hold on to them. The Bishop of Cologne, he suspects, might have something to say about one or two of the honours he has proclaimed as his own. He is even - because of the publicity it would generate, I suspect - looking forward to thrashing it out with another pretender.

I could start the ball rolling myself and use this space to announce my own accession to the emperorship, claim the duchies of Swabia, Franconia and Thuringia for my goldfish and establish a rival empire downriver at my flat in Greenwich. But I don't think my mother would go along with it.