"Chef only makes it for favourite customers," said the waitress. Leonard tried again, not impolitely, but she shrugged. "Maybe next year. Today you have something else." So Leonard plonked for a plate of tuna and lettuce.
At the moment, most people seem prepared to do anything for Mark Leonard. He is on top of the ant hill. Leonard, should you need reminding, is the tousle-haired, cupid-lipped 24-year-old with the yoof accent and the sharp suits who created such a stir last year with his call for the United Kingdom to be "rebranded". He identified five "stories" of British identity - a belief in Empire, great institutions, fair play, industrial might and the English language - and told us they were dead and done for. Those who agreed with him called him a genius. His critics called him "an insufferable wunderkind".
He had been a junior member of Demos, the left-of-centre research group, for only two years, but thanks to his astute slaughtering of sacred cows, combined with the usual media pigeonholing, he was credited with responsibility for the whole "Cool Britannia" circus. Now, at an age when most are riven by gauche doubts, he is, so they say, Tony Blair's choice as New Labour's political commissar on international affairs, having been charged with the task of running a new foreign policy "think- tank" in London.
At present it is less tank than armoured personnel carrier. The Foreign Policy Centre (FPC) is so new that its doorbell is marked by a piece of paper Sellotaped to the entrance intercom. Leonard answers the front door himself. He has a loping gait and, yes, he's wearing one of those snappy dark suits. His black hair, lightly moussed, flops down over his brow. The temporary office is small and untidy; tall piles of unsold copies of Leonard's infamous pamphlet about modernising the monarchy cover the floor. There are yellow Post-It messages stuck on telephones, and on the computer screen is some work in progress. It is a draft conference agenda entitled "Why Muslims are our Friends".
Leonard spent much of his life in Belgium, where his parents have lived for the last 18 years. He is a pampered member of the young European elite, then - and at times seems suitably unworldly. Yet his task is nothing less than to remould British foreign policy - or, as Westminster cynics put it, to help New Labour bypass the public school toffs at the Foreign Office. "Yeah," he says, with a casual laugh which must make mandarins twitch with rage. "I'm just having a really great time at the moment."
As well he might be. His contacts book of top people's numbers is as fat as British Telecom's A-K London directory. Allies range from Zeinab Badawi, newscaster and Cool pundit, to Stephen Twigg, MP; from Tony Blair's tennis partner, Lord Levy, to the former European Union ambassador, Sir Michael Butler. Leonard is important enough for captains of industry to court him over expensive dinners. The Guardian's editor, Alan Rusbridger, is said to "rate" him, and Lord Marshall, chairman of British Airways, also values his advice.
An instinct for statements guaranteed to make middle-aged Britons clutch their dewlaps in outrage has earned Leonard his title of "the modernisers' moderniser". This summer, when Leonard published the pamphlet calling for a more up-to-date monarchy - one in which the Queen Mother would queue at NHS hospitals and Prince William would be schooled at the local comp - his reputation was secured. Other targets have included the BBC World Service (should discard its "bowler hat image"), the judiciary (should be appointed by a Minister of Justice) and the Privy Council (time for it to be zapped).
Downing Street has been content to let him have his head. He is too young to have been "contaminated", as the Sunday Times put it, by Old Labour. As far as Tony Blair and Peter Mandelson are concerned, republican noises involving the Queen Mum are best made by someone who is neither a Labour MP nor a special adviser funded by the taxpayer. Blair gets the policy conceptions he likes; Leonard, meanwhile, gets to run his own think-tank. Leonard met Robin Cook only "two or three times" before he was offered the job. The call, when it came, was from one of Cook's special advisers, but word has it that Downing Street really made the decision. Of Tony Blair's involvement, Leonard will only say: "The Prime Minister is our patron."
At lunch, that weight-conscious plate of tuna in front of him (friends say he is sensitive about his waistline), he denounces the "complete crap" that has been reported so far about his new job. It is crap, he insists, that he has been inserted by Blair to keep an eye on our erratic Foreign Secretary, crap that his appointment is a snub to the twerpish boffins at Chatham House. To declare solidarity with unmodernised twerps, I order a greasy plate of egg and chips. Leonard averts his gaze.
Although the FPC is officially independent and unconnected to the Foreign Office, can a cuckoo really be tolerated in the nest? "Our work will not just be about the Foreign Office, but about all Government departments," Leonard says. "We're not there to annoy anybody." It is still early days, but a flavour of what may be to come was offered last month when Leonard said that British diplomacy should no longer be about "sending morris dancers to Oagadoogoo". It may not have upset Oagadoogoo - no such place exists - but Whitehall veterans shuddered, and Leonard's popularity in morris-dancing circles may also have been dented.
Leonard aspires, almost convincingly, to a political innocence. Even if it is an act, it makes a welcome change from the vendetta-hooked spin doctors of New Labour. With his semi- intellectual sheen (think-tanks, like hard-back books, have a kind of bogus respectability), Leonard stands apart from the Alastair Campbells and Charlie Whelans and all the other bit players in the Blairite project, with their back- stabbing and their tangible ambitions.
You could call his own rise meteoric. Then again, Leonard had good connections. Please don't call it an Old Boy network, but his father, Dick, is a former Labour MP and Fabian Society committee member who is now a Brussels-based analyst and writer. Mark, an ardent Europhile, spent most of his childhood in Brussels, speaks three languages, and studied at a school which taught him history from a French perspective. Why, then, all this sub-EastEnders speak? Leonard denies vehemently that his voice has been "artificially yobbified". "My parents recently found a tape recording of me when I was four and a half. I had the most powerful Cockney accent."
Leonard went to Caius College, Cambridge, where he read social and political science, but it wasn't a happy time. "There was a real sense of atrophy there. My college was 650 years old but a lot of the customs dated only from the 19th century. There was a mock-medieval dining hall. The college was very public school, very male, traditional and regimented. You had to be in bed by a certain time, had to buy 45 dinner tickets a term, wear a gown for dinner and listen to some bloke saying grace in Latin." Leonard wrinkles his nose, discounting such feebleness. "You couldn't even put up posters in your room windows. I had to take my Labour party poster down." On the scale of man's inhumanity to man this rates low. "Ah, but the thing that really bugged me was the way the terms were so short. You hardly had time to think."
The great thinker claims that he was expected to take a first-class degree, but in the end he managed only a 2:2, a failure which still seems to niggle him. "His intellect was a little dogmatic," remembers one Caius graduate. Another recalls: "Mark was determined not to fit in. He complains about the college gates closing at 2am, but were the porters meant to stay up all night just so he could go to bed later?"
After graduating, Leonard joined Demos in 1996, having spent two months as a trainee journalist on the Economist. His introduction to Westminster had come four years earlier, in his gap year, when he landed a job as a researcher. His office was next to that of a newly arrived MP for Hartlepool, Peter Mandelson, and Leonard got to know Mandelson's flash young assistant Derek Draper. While he is careful not to be uncharitable, he marks his distance from Draper. "I've never been a lobbyist and have never been a Government adviser," he says. "I trade in the marketplace of ideas." On paper it sounds pompous, but he speaks with humour and an engaging desire to convince. "There is no actual reason for anyone to listen to me," he admits cheerfully. "No one has elected me. Anything I say has to stand on the strength of its argument."
In some ways he is strangely conventional. This preacher of change does not, for instance, know how to drive a car. He lives in north London, in a flat in his old family house - "All of Britain is distilled into our street. Beryl Bainbridge, Julian Clary and Anne Diamond have lived there, and there's a bus driver two doors down from me" - but takes holidays at his parents' villa in Burgundy. He has been with his girlfriend, Gabrielle, a jewellery designer, for 18 months. "Do you think you might settle down with her?" I ask, like a nosey old aunt, fully expecting him to snort derisively, as most 24-year-olds would. But he comes over all serious and indicates that, yes, soon it might be time for things to be placed on a formal footing.
His own family is the most important thing in his life. His maternal grandparents were German Jews who fled the Nazis, hid in France and had a horrific time. "They weren't religious, yet they were aggressed because they were Jewish," says Leonard. "I am not religious either. My route to Judaism is through the Holocaust." It means that he sometimes views history "with a persecution complex". His late Jewish grandmother helped to rear him. She would feed baby Mark, tell him stories, take him for walks and, from the age of three, read him Shakespeare. It sounds like a fine unmodernised childhood: one wonders whether, at some stage in the future, he'll write a pamphlet about child rearing and suggest a return to traditional values.
Speaking of which, he argues that tradition is "great when it adapts - but it stops being good is when it becomes a prison". But don't traditions and custom form a communal identity? "Obviously people need to be rooted in their national identities but you can't forget where you want to go to. The Dutch are incredibly sure of their own identity and they are patriotic - they even have a Royal Family - but they are very relaxed about regionalisation and European integration. Britain could be like that, too."
The only problem with having known such success so early is what to do for an encore. What next? Does he want to become an MP? Three times I ask him; three times he dodges the question. For one so identified with re-branding, he now feels, ironically, that he needs some re-branding of his own image. "I really hate all the Cool Britannia stuff, you know," he says. "I really do." More than the rest of us? Impossible.