For the Oxford Union, the recent controversy over the BBC's invitation to Nick Griffin to appear on Question Time is nothing new. In 2007, there was a similar eruption of moral outrage when then union president Luke Tryll invited Griffin to speak in a debate about freedom of speech, and I was assigned to debate against him.

The debate made headlines around the world. A variety of senior politicians wrote angry articles complaining that by inviting Griffin we were legitimising him and his party. The evening Griffin came he and the audience had to fight their way through hordes of screaming protesters to even get into the building.

Inside, I found myself facing a figure who in no way reflected his media image. The most surprising thing about Griffin is how intelligently, and even normally, he comes across. We, in common with most of the media commentators then and now, were expecting him to be a bitter, resentful, hate-filled Little-Englander who would stand up and rant about blacks, Asians and homosexuals. His supporters certainly lived up to these expectations – one of his bodyguards actually told me I should leave the country and go back home (I was born in South Africa). Griffin, though, is much cleverer than that.

The Oxford debate was about the limits of freedom of speech. Griffin gave an intelligent, thoughtful and well-researched talk about restrictions on free speech throughout western history, drawing on Voltaire, Locke and Mill. He spoke about Galileo being persecuted by the Catholic church for his heretical scientific theories, and compared him to the modern far right, demonised and prevented from speaking by a liberal metropolitan elite, blind to the fact that its principal concern – the alienation of the white working class from the political process – is a real, and dangerous, problem. He drew a picture of a middle-class establishment concerned only with London and the south-east, and desperate to marginalise those sectors of society they considered un-PC. His speech was accompanied by the low, rhythmic sound of chanting coming from the protestors outside: "Kill Tryll, Kill Tryll, Kill Tryll," they chanted in impeccable Received Pronunciation. To most of the audience in the room, it was they who sounded like the extremists, and Griffin who appeared the voice of reason.

Yet when I pushed him to talk about race, challenging him to really compare his views on blacks, Asians and homosexuals to Galileo's theories on heliocentrism, the facade broke down. Under pressure, not from an angry rabble but from a small squad of world debating champions, he reverted to snarling, racist type. He claimed that in the north of England gangs of Asian men roamed the streets kidnapping and raping white girls. He claimed that most non-white immigrants would welcome repatriation to their ancestral homelands. He demanded an end to mixed-race marriages, and questioned the accepted history of the Holocaust. Under a barrage of criticism and probing questions, his false air of respectability disappeared, and he revealed the racist reality that he prefers to keep hidden.

And herein lies the lesson for Jack Straw, Sayeeda Warsi and Chris Huhne. The way to beat Griffin is to force him to talk about the issues and policies he doesn't want to talk about. He will not, as is sometimes assumed, politically hang himself if merely given enough rope; the panelists and audience will have to do it for him. On Question Time, he will try to speak the same, reasonable-sounding language he spoke at the Oxford Union; as we did in 2007, the panelists must force him to speak the angry, racist language of demagoguery that he prefers to keep for BNP supporters' meetings. Or, failing, that, they should just stick a camera in front of his bodyguards.