Nigel Farage hasn't changed a bit, and yet everything is different. He suggests we meet in a village in Stockton-on-Tees for a "lovely pub lunch", which turns out to consist of three pints of ale, lots of cigarettes, and no food whatsoever. Already he is hoarse, and it's only day two of his grand tour of the north. "I gave myself a stern talking-to yesterday on the way up to Sheffield, and told myself not to overdo it," but then 600 supporters turned up for Ukip's launch rally – "It was like a Billy Graham meeting!" – and so of course the evening got a bit out of hand, and he got to bed at "oh, you know, aherm, five past". But straight after lunch he bounds back on to the tour bus – "I've got to go and liberate a country" – and sets off for a nearby town walkabout, which winds up in another pub. Dressed in a startling mustard corduroy ensemble, and mobbed by a scrum of local media, he looks like the happiest man in the world.

But by his normal standards, this is an operation of military discipline. If all goes to plan, his party is four weeks away from pulling off "one of the biggest upsets in British political history", and its leader is taking no chances. If Tony Blair, as Roy Jenkins once said, approached the 1997 election "as if carrying a Ming vase across a polished floor", then Farage is inching towards 22 May like a barmaid balancing an overloaded beer tray.

He now travels with a security detail of four big men in suits and earpieces, which is costing Ukip a fortune and driving him mad. "I'm a completely free spirit, and of course these guys are used to meticulous timing, so I hate it." But he was whacked over the head with a placard in Margate recently, "and if I hadn't had them with me I'd have been in hospital". He watched the first televised debate with Nick Clegg, "And I thought I was irritable. Bit red. A touch sweaty, possibly. So I went into training for the second debate. I did, I did! Cos I thought, this is really important, this actually really bloody matters. I've got to get this right, I've got to get myself in shape for this." For four whole days he didn't drink, he went walking on the North Downs, had a few morning swims, "and actually got myself into a position where I wasn't knackered, I wasn't boozed up, and I was actually fit to do the debate. And I think it worked." He did have one glass of red just before the second debate, but only really on principle. "We got to the green room and there was just mineral water! I said: 'We can't be doing with this sort of thing, we want a bottle of white and a bottle of red or I'm not going on.' "

He has also had to learn how to say as little as possible – something else he clearly hates. "But we are fighting this election on who governs our country," so Farage point-blank refuses to specify a single Ukip policy before 22 May, other than withdrawal from Europe ("Oh, and bringing back grammar schools, you can have that"). He even refuses to rule out any of the policies that featured in the party's 2010 manifesto, a document he describes as a "load of late-night ramblings" and has never actually read. Will he not even rule out one of its more memorable pledges, the compulsory dress code for taxi drivers? "I've burned all 400-odd pages of it, burned it all, so I'm not prepared to discuss any of it."

Self-censorship doesn't really suit Farage, so he resorts to suppressed giggles and frantic gurning whenever he won't let himself answer a question, casting himself more as a mate planning a surprise birthday party than a slippery politician evading scrutiny. When it comes to Ukip policies already on the record, if he can't defend them then he says they're simply nothing to do with him. For example, the party now requires all candidates to confirm that they have no skeletons in the closet that could cause embarrassment to the party, but what does that actually mean? "I've no idea. 'Could cause an embarrassment' could mean almost anything, couldn't it? I just hope I'm exempted." Are any candidates likely to disqualify themselves on the basis of this clause? "Depends how ambitious they are, I suppose. Oh, I don't know, I'm surprised they used the phrase to be honest. But as I say, I don't run everything."

Nigel Farage is enormously pleased with the Ukip posters he launched this week – if anything 'we should have gone further, really' Photograph: Tom Maddick/Ross Parry

Farage knows he can't rely on the party's original support base – "very middle-class, very below the M4, ex-military" – to deliver victory next month. So the privately educated ex-stockbroker from Kent is now chasing northern urban voters, and claims his party has become working-class. "What we've got to do in the next four weeks is rattle the Labour party. That's the big job for Ukip now. The middle-class Tories in Wiltshire have already decided whether they're going to vote Ukip on 22 May or not – and a lot of them are. So for the next four weeks the focus is entirely on the Labour vote. If we're going to win these elections, those are the voters we want to get."

He is probably wise to steer clear of policy detail, for Ukip is now such an ideologically incoherent bandwagon that policy is no longer the point. Some of its supporters want to make dressing up for the theatre compulsory, while Farage is a libertarian who breaks the law "regularly" and says: "I'm not for laws. We need a minimum of laws. I hate big government, I hate being told what to do on a personal basis." But even he is a bit wary of libertarianism now, "because I've seen some of the loonier elements. A bloke took over our youth wing and called himself a libertarian, and he's for, I don't know," he shudders, "bestiality!" But he thinks he can get away with sidestepping all these contradictions, because none of them will matter on 22 May. "People are voting for difference. I think there's a feeling among our supporters that we speak for them and no one else does."

One problem for Farage is how to maintain his image as a chaotic amateur while communicating quite a shrewd electoral strategy. The key to winning next month, he explains, is to get the non-voters out. "Over one in five of our votes already come from non-voters. And in European elections only a third [of the electorate] vote, so it's a massive marketplace. And the more urban the area, the lower the turnout." If Ukip can beat Labour to first place – and the gap has closed to just a couple of points – "the grassroots pressure on Ed Miliband to promise a referendum will be irresistible. We need to get British politics into a position where, whatever the outcome of the next election, we get a referendum."

He is enormously pleased with the Ukip posters he launched this week, one of which features a gigantic pointing finger beside the words: "26 million people in Europe are looking for work. And whose jobs are they after?" But as the BBC's political editor asked Farage this week, if he's so worried about Europeans taking our jobs, why does he employ his German wife as his secretary, at taxpayers' expense? "That particular line of argument from Nick Robinson was pathetic," he scoffs. "If that's what he wants to focus on, then it's a pretty poor reflection of him and the BBC in my opinion. I think it's astonishing, simply astonishing. I mean, it just shows you how trivial the whole thing is." He has to employ his wife, he says, because nobody else could possibly work such antisocial hours so closely with him.

But other MEPs manage to employ secretaries to whom they are not married, don't they?

"How can you compare my life to any other MEP? I mean, come on, it's crackers, isn't it? Look, other MEPs do five days a week in Brussels and pop home for weekends. I'm working seven bloody days a week, all the hours God sends. If you include the socialising, it's over 100 hours a week." His wife is not the only immigrant he has employed, it emerges; he has hired other Europeans, "because they had specialist skills – languages in particular". But he says British people should always try to employ a Brit over an immigrant wherever possible. "And is that discriminatory? Maybe it is." Is that what he would always do? "Of course I would, yeah."

The posters have been condemned across the political spectrum as racist, divisive and ignorant. He knew the finger-pointing poster would be controversial, but can't for the life of him see how it could cause offence. "Shouldn't do. We should have gone further, really. What we could have done is say we've opened the door to 425 million people who are after your jobs. That would have been stronger." I suspect the decision not to had something to do with criticism of a notorious Ukip leaflet last year, which warned: "The EU will allow 29 million Bulgarians and Romanians to come to the UK." As the combined population of both countries did not, in the event, relocate to Britain on 1 January, you'd think he might be embarrassed about the baseless alarmism, but not a bit of it.

"Not at all. Two reasons, really. I think we don't know the true figures yet. And there's also the quality debate. People hate talking about this, but if you look at the Met crime figures for Romanian arrests, there have been 28,000 in London in the last five years. Is there a problem? Yeah. There is a problem." Is he saying there is a culture of criminality among Romanians? "Bound to be. You have to go and see it to understand it. I've visited camps in Romania and Bulgaria, I've got a pretty good understanding." Should British people be wary of Romanian families moving into their street? "Well, of course, yeah."

The poster campaign was funded by Ukip's main donor, the businessman Paul Sykes, but I'm not convinced that relations between the two men are quite as harmonious as Farage says, because he looks a little strained when I mention Sykes' name. The pair considered dozens of different poster designs, he says – so who had the final say? "Me." Sykes would have paid for posters he didn't choose? "We agreed. But if we hadn't agreed, we wouldn't have had them." After a brief pause, "I'm not for sale," he suddenly barks. "I'm not for sale." But Sykes' money is crucial, isn't it? "Well, his money makes a huge difference to us, of course it does," he says briskly. "I get on incredibly well with him, we're pretty eye-to-eye on lots of issues, and getting agreement on this campaign and how to do it has been very easy. Very easy." Yet Sykes still hasn't promised to bankroll Ukip's general election campaign. He says he's waiting to see how the party performs in May, so I ask Farage what Ukip has to do to secure Sykes' backing for 2015. "Well, you'd better ask him that." Hasn't Sykes told him? "No."

The party's finances are a perennial headache for Farage, and like most Ukip MEPs he has donated a chunk of his own money – £11,000 last year, £4,000 this year. It was recently reported that the party is drawing up an MEPs' charter, demanding a compulsory donation from each MEP of £50,000, but Farage says it will stipulate only a "reasonable sum", which in his case will be "two to three grand". He recently described himself as "broke", but has a household income of just over £100,000. How can Ukip be a working-class party if it will only accept candidates wealthy enough to donate thousands? "They wouldn't have their jobs as MEPs if it weren't for Ukip. They're not getting their jobs as individuals."

Money has become a tricky subject for Farage lately, since it emerged that supporters donated for free an office that he lists as a significant running cost. He insists he has done nothing wrong, so I ask if he can give voters a guarantee that every penny he's received from the EU has been spent correctly.

"I can guarantee one thing. That I haven't done it for personal gain. But how I've spent my time and money, and whether I've spent it because I'm an MEP, or because I'm Ukip, I would suggest to you is a very grey area. It's a difficult divide. I've made no bones about it that I would use the wherewithal provided by the European parliament to go round Britain and campaign against Britain's membership of the European Union. I think I'm just about within the rules. I think I've kept just the right side of the line. Albeit pushing right up to it, sure."

And if he has strayed over the line and broken the rules? He doesn't think it matters. "I mean, given the abuses for personal gain that have gone on with expenses in Westminster, I don't think the general public are that interested in whether I've strictly observed the rules on what is campaigning and what isn't. We always knew these criticisms would come at some point, but I have a completely clean conscience. If someone in Brussels wants to martyr me for that, then, well … well, they won't, they won't."

He told the Today programme last week that he would be happy to have his expenses independently audited, but he is now keen to correct this. "No, I didn't. I said if every other British MEP wants to then I would. I mean, I am not going to be one out of 73 that is held up as an example of all that is wrong with the European Union. After all, I've been saying that myself for years, so this is absolutely ludicrous. If all 73 people want to go on to a new regime, then of course I'll do it, but to be singled out in this way is frankly ridiculous."

The other charge levelled against Farage is that he can be a hot-headed bully, so I ask when he last raised his voice at a colleague. "Um, that's a very good question. I had some sharp words with somebody two days ago. I told him to sharpen his effing act up. He said to me today, thank you for that. But I very rarely lose my temper. You know, really lose my temper. Very rarely, very rarely." What about a barky growl? "Oh, I do that quite regularly."

He is one of the jolliest politicians I've ever met – exuberantly self-deprecating ("Why would I take paternity leave? I'm absolutely useless!"), quick to laugh, great fun, and uncommonly at ease in his own skin. But I would guess that, when provoked, he can go off like a bomb. Interestingly, he says of the Clegg debates: "He really, really tried to dig me in the ribs in that second one, but I knew I was calm, I knew I was in control. I think if he'd gone down that line in the first debate, I might well have snapped. Which I would have enjoyed – but probably no one else would have."

The day after our meeting, Farage had to suspend one of the stars of Ukip's first ever party political broadcast, a council candidate, after racist tweets from his Twitter account were exposed. When I interviewed Farage in January last year, he was still a maverick figure on the political margins, and for all his charisma, it was hard to see how he could protect himself from coming to grief through his colleagues' bigotry and battiness. I still can't work out whether what he's achieved since then says more about the failings of the traditional Westminster class than it does about Farage's own acumen. But his apparent immunity to any amount of Ukip scandals is looking less and less like just good luck.

Nigel Farage knows he’s lucky, having narrowly escaped dying when a plane towing a Ukip banner crashed on election day 2010. Photograph: Neil Hall/INS News Agency Ltd

Then again, as he points out himself, he is a very lucky man. It's surprisingly easy to forget that on election day in 2010 Farage very nearly died in a plane crash, when a Ukip banner the light aircraft was towing became tangled in the tail fin. At the time it seemed almost like a bad joke – a metaphor for an eccentric political career nose-diving into oblivion – but obviously not to Farage. "Well, it was horrible. Yeah. It's one of those things I still think about." He can remember "every single millisecond of it", and thought he was going to die. What went through his mind? "All sorts of things that shouldn't have done," he laughs. "I'm not going to tell you." Go on. "Well, I thought a bit about things I'd done well, things I'd done badly. I thought about all the different girlfriends I've had, you know, about different forks in the road at different times." Was he seized by regrets? "Well, we all have regrets in our life – but I'm married, you see," he laughs, "so I can't answer that."

He thought about phoning his wife. "But then I reasoned that probably that phone call would haunt them." If he had called, he would have said: "Sorry I've been such an appalling husband and a not very good father. But how would that help? It wouldn't have helped. So I just sat there quietly. And then when the end comes, and you're careering towards the earth, there's almost a sense of resignation. Let's hope it's over quickly."

He came to rest upside down, his head two inches from the ground – "That was the difference between snapping my neck and not." Covered in fuel oil, he thought he was about to burn to death. "And that was terrible. And I thought, nobody will ever know I survived this crash. And then, after a few minutes, it hadn't caught fire, and I began to think it might be OK. But I couldn't breathe, I just could not breathe. And I thought, you know, all those years of smoking – if I get out of this I'll never touch another cigarette as long as I live, I'll be a really good person. If I get out of this I'm going to live such a good life, I'm going to be such a good person." And has he kept any of his promises? "No! None of them."

He thinks that since the crash, "I'm a little bit more aware of others than perhaps in my worst moments I would have been before." Awareness of others has never been his strong point, he chuckles softly. "I think it's been a weakness. But I wouldn't be doing what I'm doing otherwise. To do what I've done in this job, I think you have to lack self-awareness."