Police have been checking whether David Cameron's chief spin doctor, Andy Coulson, knew that a journalist working for him was illegally hacking telephone calls when Mr Coulson was a tabloid newspaper editor. Had it been Kelvin MacKenzie – the most successful and notorious boss in what used to be called Fleet Street – there would have been no need to ask. Everybody would have known he was in it up to his neck. He says so himself.
"Me, I definitely would have known. I would have been all over that information and I would have loved every second of it. I would have been sitting there listening to the voicemails, reading the texts and literally rolling up, killing myself laughing, and also being sure I wasn't going to be sued for libel," Mr MacKenzie announces, with that flamboyant shamelessness that prompted Piers Morgan, whose mentor Mr MacKenzie once was, to describe him as a "dangerous genius".
That is not to say for one moment, however, that he is accusing Mr Coulson of having lied about what he did or did not know. "Coulson is a very reserved, very cool, very detached character, nothing like me at all," he continues. "He's nothing like most editors, to be honest. On that basis I am absolutely certain that he wouldn't have shown the slightest interest in where the stories were coming from, or got the pleasure that I would have had out of sitting there reading these texts. That's a personality issue. So I believe 101 per cent that he hadn't got the vaguest idea where the source was."
We meet in a Soho hotel, a fitting part of town for a man who wallows in his reputation as intellectual low life. You cannot debunk Kelvin MacKenzie, any more than you could parody the famous headlines he produced during 12 years as editor of The Sun, such as "Freddie Starr Ate My Hamster", or his message to Jacques Delors, then president of the European Commission: "Up Yours, Delors."
Mr MacKenzie makes a lot of people angry, and even now he would have to take care when entering a pub in Liverpool because of what The Sun said about the victims of the Hillsborough disaster 21 years ago. His career has thrived on provoking a reaction. But when he is attacked, he has an almost invincible line of defence – that he has never claimed to be any better than he is.
He shares a curious feature with other outspoken right-wing celebrities, such as Jeremy Clarkson and Andrew Neil, that his head is unusually large for his body. Sitting down, he looks enormous; standing up, he is of normal size. He talks and talks, then stops suddenly and announces that he is being boring, before setting off again. His opinions are in "shock jock" terrain. He does not like paying tax, despises most politicians, does not trust officials who work in offices for the Government or a council, he is anti-EU, anti-immigration, and anti-BBC, and thinks that Rupert Murdoch is a great man whose greatness is unfairly unrecognised.
He also has a rough-house charm that makes him easy company. Though he has been very famous and very notorious, and has made a considerable amount of money, he is appealingly short on pomposity. He is also, curiously, despite his cultivated public image, polite. If there was a credible movement in Britain equivalent to the American Tea Party, Mr MacKenzie would be part of it. What such a movement lacks, in his view, is credible leadership. He would like to see a political leader emerge from the people, preferably someone with everyday experience of dealing with difficult economic conditions, such as a shopkeeper. He is not offering to assume the mantle of leadership himself. He did dabble in politics once, but thinks it was a mental aberration.
He ran for a seat on his local council two years ago on a "Red Mist" ticket, as a protest against parking charges, knowing there was no chance of winning, and did reasonably well. That seemed to go to his head, and when the Tory MP David Davis caused a by-election in his Haltemprice and Howden constituency, near Hull, Mr MacKenzie floated the idea of running as a Red Mist candidate, apparently thinking he could win. But his name never got on the ballot paper. It was rumoured that Rupert Murdoch told him to pull out. Mr Mackenzie denies this, saying that he knocked himself out of running when he opened his mouth and described Hull as an "absolute shocker – beyond shocking". He realised then that politics was not for him.
"I basically went mad," he recalls. "Taking a train up and down from Hull for the next couple of years, until I got absolutely stuffed at the next general election, was clearly not something I wanted to do. I wouldn't be a good MP because I would be vaguely hostile to the constituents. If they came to me about their benefits, I'd say: 'Well, why don't you go and get a job?' I don't think that's the answer they'd be looking for." Still, he thinks he understands better than the politicians what the people feel. He claims: "The political tribes and their views don't cover how most of the electorate feel about issues. Supposing we said we're going to have referendum politics, we're just going to trust the people on certain areas like Europe. We've ended up being forced to give the vote to a killer who has denied the vote to his victim because she is now dead. How bizarre is that?
"Why don't we have a Prime Minister, or Deputy Prime Minister, or Leader of the Opposition prepared to say: 'We're not going to bring this in and the court of human rights can fine us as they like, and we're not going to pay'?"
There is, however, a glaring inconsistency in his "up yours" view of the European Court of Human Rights, because his friend – and fellow right-wing populist – Jon Gaunt is engaged in a legal battle about his right to broadcast his opinions, having been sacked as a TalkSport radio presenter for calling a councillor from Redbridge, east London, a "Nazi".
"The court of human rights will find for Jon Gaunt," proclaims Mr MacKenzie, unfazed that this is the same court he had been denouncing in a previous breath. "They'll say: 'Anything you say down the pub which doesn't break the law, you are allowed to say without balance on radio or television.' I'm in favour of the court of human rights on this issue."
Mr MacKenzie also believes that if the British people were given a referendum on leaving the European Union, and even if the economic consequences of leaving were clearly set out, the result would be close.
He applauds the vast multicultural city that London has become. "The melting pot that is London is much more multi-rainbow than even New York – it's incredible," he says. Yet, when challenged, he has to admit that if the referendum culture that he advocates had existed in the late 1960s, when Enoch Powell was calling for Asians and African-Caribbeans to be deported, London would not be a melting pot at all.
"That's a good point," he concedes. "I do accept that some liberal movements would be delayed, but no liberal movement worth its salt would be scuppered. You would have to define what referendums would be allowed and how many – perhaps once a year, or something, on the big issues."
Inconsistencies such as these would be a serious drawback in a politician, but they are of no account in Kelvin MacKenzie's highly successful brand of popular journalism.
He was born into the trade. Both his parents were journalists. His father ran a news agency, MacKenzie of Dulwich. Kelvin was phoning his father's or his mother's words over to copy-takers when he was still at primary school, and got his first job on a newspaper at the age of 17.
His brothers, his children and almost everyone in his extended family works in the media. He loves the life, but does not profess to have an exalted idea of its ethos. There was a famous incident when the Mail on Sunday caught him on holiday in the Caribbean with a secretary 20 years his junior. Some time afterwards, his 38-year marriage ended in divorce. He has since remarried.
Other people in Mr MacKenzie's line of work who have been caught out suddenly become rather precious about the importance of personal privacy. He never wavered in his view that those who dish it out should know how to take it – though he confesses that there was an occasion in his long career when something written about him was so obviously false that "I got a bit up myself" and consulted a lawyer, who advised him that though the words were indubitably libellous, they could not be said to damage his reputation because he had no reputation.
"I think that if you are a tabloid editor, then you deserve everything you get, frankly. I have no right to privacy," he says. "You have got to understand where you lie in the ladder of life. I have absolutely no doubts about where I am. It would be somewhere near the bottom. It may be at the bottom. I always thought when editing The Sun that the greatest journalistic act I could perform would be actually exposing myself so you'd end up with a headline: 'Sun Editor in three-in-a-bed scandal'. And you'd start with an intro: 'Kelvin MacKenzie wept with shame last night as it was revealed...'
"I think the idea that editors are in some way part of the establishment is just plain wrong," he adds. "I sort of drifted in and out of that. The idea that the Prime Minister would ask you what you thought about something is truly, truly absurd."
It may be absurd, but it happens. David Cameron is the latest Prime Minister to open Downing Street's doors to Kelvin MacKenzie, who was allowed in the week before last to rant for a several minutes on themes familiar to those who follow his column in The Sun: about why Sky News should be allowed to let its news presenters hold opinions and air them, just as the right-wing presenters of Rupert Murdoch's US channel, Fox News, do; about how deeply he, Kelvin, objects to contributing to the salaries of BBC journalists who have been out on strike; about how the BBC and ITN are not impartial at all but controlled by "on the one hand this and on the other hand that" lefties who have never run a small business or known what it is to be skint; and about how, if you watch Channel Four News, "you think you're in North Korea half the time".
"I spouted on like this at Cameron for about 10 minutes, and he indulged me," Mr MacKenzie said. "To be honest with you, as he shut the door, I think he felt to himself: 'Well, there goes that nutter'. But at least I was able to say it. I rather like Cameron, to be honest. I think he's rather a decent bloke."
MacKenzie is now 64 and says that, as in the old Beatles song, he occasionally asks himself: "Do they still need me?" But he has no doubts that journalism has a thriving future, and makes no apology for his part in making the trade what it has become.
He is thinking that when he reaches 67, he might throw a party to celebrate his first 50 years as a journalist, and on the invitations he is thinking of printing the legend: "50 Years of Bringing the Game into Disrepute."
I hope he asks me, not because I necessarily want to go, but I would like to have that invitation framed on the lavatory wall. You cannot debunk Kelvin MacKenzie, because he knows himself, and he is shameless. Utterly shameless.