My executive producer, Victor Lewis-Smith, has always believed in biting the hand that feeds him – right up to the armpit. When he was a BBC producer, he told his head of department to insert the whole of Egton House, the BBC building with the sharpest edges, into a place not recommended by the Royal Society of Proctologists. A few months later, he dialled Radio 4's Call the Controller phone-in while posing as a member of the public to berate his boss for the dire quality of his network, emphasising the dismal quality of his own programme. Just last month, so he told me, he found himself in an office with the chief executive of Channel 4, calling him "a spineless cunt" for not broadcasting a documentary we'd completed a few months ago. And his Tourette-like outburst seemed to do the trick, because Keith Allen Meets Nick Griffin is finally being aired on Channel 4 tomorrow evening.

Making a documentary about the BNP leader was always going to be contentious, and you may wonder why we ever embarked on it. The programme was born out of us watching how the media covered the notorious edition of BBC1's Question Time that featured Griffin shortly before the last election. The media declared that he'd been humiliated, and that the BNP had been trounced in the subsequent election. Yet the BNP actually almost trebled its vote, from 192,746 in 2005 to 563,743 in 2010. Why were the media insisting that the BNP had been annihilated, rather than focusing on the disturbing fact that well over half a million people had voted for them? And was their relentless vilification of Griffin, while pleasing to their liberal audience who'd never vote for him anyway, paradoxically making him more attractive to those alienated sections of the population who just might?

When we proposed Griffin as our next Channel 4 documentary subject, they were enthusiastic, but warned us that the BNP were unreliable negotiators. And so it proved, as we embarked on a lengthy and often bizarre cat-and-mouse game with Griffin's PA, who repeatedly declined our offer, before eventually inviting us to Brussels, where Griffin sits as an MEP, for filming. I was suspicious. I have a well-publicised history of supporting anti-fascist causes, and my daughter Lily wrote an anti-BNP song entitled "Fuck You". So was their offer genuine? That's a rhetorical question because, on the agreed day, with our film crew hurtling towards Belgium on Eurostar, the BNP sent us a terse email saying "no interview – your documentary is cancelled", which I assume was their idea of a very expensive joke.

Having committed most of our production budget to the shoot, we decided to continue anyway, and spent an extraordinary day inside the European Parliament building looking for the portly BNP leader. When we spotted him, he scurried off towards his office with me and a cameraman in pursuit. And when we realised he had locked himself inside and called security, we asked him through the door why he was behaving in such an extraordinary fashion. When security arrived, one of Victor's Tourette outbursts surfaced as he began arguing so vociferously with the officer that the entire crew were nearly thrown out of the building. But there was method in Victor's behaviour, because he suspected that the BNP wouldn't want a "Griffin involved in fracas at EU Parliament" story appearing in the press. And sure enough, an hour later, the BNP phoned us and agreed to give us two interviews.

They form the backbone of the programme and may well displease some viewers, because Griffin didn't reveal himself to be either a monster or a madman, but expressed his political views – many of which are utterly loathsome – in a logical and coherent manner. His party is no longer racist, he claimed, because people of all ethnic backgrounds are welcome, so long as they subscribe to core "British" values. I did point out to him that the BNP was forced by law into making that change, and, in reality, BNP membership remains as remorselessly white as ever.

Under Griffin, the party has become obsessed with anti-Islamic rhetoric, replacing racial intolerance with religious intolerance. Griffin even claimed during the interview that "the Koran isn't primarily a religious tract: it's a handbook for conquering other people's countries". This is a deliberately provocative statement designed to play on the lowest prejudices in "white Britain" against Muslims.

Although I bluntly refuted his statements and called him "a Nazi... lacking in humanity" to his face, I let him speak. Nor did we subsequently edit the footage to make Griffin seem incoherent or foolish, and I was reminded of another programme that I made with the God Hates Fags sect in Topeka, Kansas, who expressed their vile homophobic views with equal logic and precision. Making them appear stark raving bonkers may be an audience pleaser, but that doesn't fairly represent who they are and what they think, as our documentary did. Of course, you may think I'm biased in this matter, but I've just checked with a former TV critic – his name is Victor Lewis-Smith – and, no, he tells me that my opinion is spot on.

Having recorded and edited the Nick Griffin documentary, I discovered to my chagrin that the problems were only half over. Channel 4, which had been enthusiastic about the idea of me interviewing Griffin, was discomfited by the reality and repeatedly asked for changes and additional lines of didactic narration. When one commissioning editor completely misunderstood a sequence about Muslim women in which Griffin was unexpectedly defending them as victims of male violence, she gave such a knee-jerk reaction that I assumed she must have trained with Galvani.

Eventually, after much patient explanation, Channel 4 said it was pleased with the documentary, but it then sat on a shelf as summer turned to autumn and then to winter. Hence Victor's outburst of a month ago, as a result of which the programme will finally grace – or disgrace – your screen tonight.

I'm primarily an actor and only make documentaries when I see a story others have missed. My previous Channel 4 Keith Meets... documentary was with the late, great Keith Floyd, who curled up on the sofa and died two hours before the programme was transmitted.

Whatever you think of the way we've dealt with Griffin, I'm sure there's one thing we can all agree on: surely we all sincerely hope that a similar fate does not befall the subject of tomorrow's documentary.