An excitable man is ranting about what he sees as government hypocrisy, through a megaphone outside the Houses of Parliament. Nothing unusual about that - except that the man is standing beside a lorry emblazoned with a huge blow-up of a pornographic article written in 1980 by the person who is now the Prime Minister's press secretary, Alastair Campbell.
While next to him a Campbell lookalike with a kilt and bagpipes cavorts with a scantily-clad blonde, the man bellows on about free speech, highlighting the Government's proposed censorship of a French porn channel.
Just another normal day at the office, then, for comedian Mark Thomas, the man for whom the phrase "megaphone diplomacy" might have been invented. As Geoff Atkinson, the series producer on Channel 4's Mark Thomas Comedy Product, puts it: "This stuff is not sophisticated. It's like doing brain surgery with a hammer."
As we stride towards Downing Street to deliver a petition demanding that the porn channel be kept open (at least long enough to broadcast a re- enactment of Campbell's steamy story), Thomas says, with a mischievous chuckle, "My series is a political version of Noel Edmonds' Gotchas. The aim is to annoy people. It's not the sole aim, but some people just deserve to be annoyed. I hope to get people who aren't normally got.
"The whole point of my stunts is to pull the rug out from someone who doesn't want it pulled out," Britain's last remaining political stand- up carries on with a glint in his eye. "What I do is offensive, deplorable and morally unjustifiable - except for the strong moral reasons that justify it. What I do is bad; the reasons why I do it are good."
Like a war veteran flourishing old campaign medals, the 34-year-old comedian goes on to list the people who complained about the last series of The Mark Thomas Comedy Product: Glenda Jackson, David Amess, Jerry Hayes, William Waldegrave, McDonald's, Rothschild, Yorkshire Water, British Gas... He was even issued with an Intent to Serve a D-Notice by Nicholas Soames. This man gets up more noses than cocaine at a rock stars' convention.
But doesn't Thomas ever tire of getting on people's nerves? Apparently not. Like a placard-waving student activist, he remains fiercely proud of his ability to annoy. He is in constant, often heated, dialogue with C4's lawyers. Thomas recounts with relish how, on the night of Princess Diana's funeral, his near-the-knuckle material about her at a comedy club caused such uproar in the house that the bouncers advised him to come on stage in a crash-helmet the following night.
"I'm Channel 4's token rebel," he claims, with evident glee. "Whenever people ask the channel, `Why have you got so many American imports?', they say, `We do a lot of other things that other channels wouldn't touch.' I'm one of the things that always gets wheeled out."
The other charge levelled against him is that he tells people what to think. "Any form of entertainment tells you what to think," he counters. "Someone has made a conscious choice about what they want you to see. Someone actually sat down and said, `The Shane Ritchie Experience - this is what they want.'"
Although political, Thomas can't stand politicians; he doesn't need the megaphone to emphasise his feelings about them. "I naturally loathe all politicians," he asserts. "It's the fact that they all have to be so duplicitous. The Mother of Parliaments? That's just an insult. Mark Twain said that the last person you'd want to see in the White House is the person who wants to be there. Where do you want me to start with the Labour Party? Indonesia, Malaysia, the control of interest rates - there's just three reasons why they're the most awful bunch of corporate knob-shiners."
So where does Thomas align himself politically? "I'm more of a surfing, cosmic-vibe-searching, non-drug-taking, non-smoking, non-drinking, Kenzo- suit-wearing anarcho-liberal," he babbles in his distinctive rat-tat-tat delivery.
The spouting may sound polished, but no comedian is ever going to bring down the Government. Atkinson acknowledges as much: "Look at Spitting Image. David Steel did suffer a bit from it, but you'd be foolish to think it had a radical effect on people in general. The best you can hope for is to make people inquire about things. I remember when I first read the Monty Python book, tears were running down my face. That's the thing that drives us, rather than thinking we'll change the world."
Thomas is a one-off: opinionated, obnoxious, over-the-top, but undeniably original. Who else would have dared drive round to William Waldegrave's house in a tank done up as an ice-cream van, ring on the doorbell and ask for an export licence to Iraq?
But for all his bravado and his balls, Thomas's brand of overtly political comedy still seems as unfashionable as William Hague in a baseball cap. Thomas himself doesn't care; he has no time for labels like "political comedy". "It's just journalists running around saying things like `Brown is the new black' or `Oasis are the new chocolate mousse'. In my view, everything is related to politics if it's about how we operate in the world."
C4's commissioning editor for entertainment, Graham K Smith, sees a virtue in Thomas's unashamedly political approach. "There is something old-fashioned about Mark," he concedes, "but in the first year of the Blairite Revolution, where everything has to be squeaky and new, there's absolutely nothing wrong with a bit of old-fashioned. The attitude that just because something's new, it's good, is bollocks."
Smith even goes so far as to venture that political comedy might yet become trendy again. "The 1970s are very fashionable now. Mark is bringing back the values of the 1970s - flying pickets, confrontation, agitprop and revolution." Smith is just praying Thomas can do so "without incurring a pounds 1 million fine for Channel 4."
To prove the point, the moment Thomas steps inside the foyer of C4's headquarters after the Campbell stunt, he is buttonholed for an intense chat by an earnest-looking man carrying documents. It's the in-house lawyer
"The Mark Thomas Comedy Product" returns to Channel 4 at 11pm tonight