The author and 9/11 conspiracist, Ian Henshall, announced his departure from the big truther's bash on Monday evening, by thrusting his bald head into my face, twice calling me a "f---g scumbag" and then flapping off into the wet night. It was a typically careless exit.

We had both been part of an audience at the Royal Institute of British Architects for Richard Gage, who is the American front-man for "Architects and Engineers for 9/11 Truth".

Mr Gage had delivered a two-hour slide- and film-assisted presentation which convinced most of the 200-odd audience that they were right to go on believing that the 2001 attacks were an inside job, involving secret, controlled demolitions timed to go off at the same time as planes were flown into the World Trade Centre.

I was there because I take an anthropological interest in conspiracy theories. Mr Henshall was there to reinject himself with the "truth".

Gage's lecture over, I got up to ask him some questions, none of which he felt qualified to answer. A few minutes later, Mr Henshall began to shout very loudly and very clearly, gesticulating in my direction. His complaint was that I had "libelled" him some years earlier by calling him an antisemite in the Guardian. Seeing that I was nonplussed, he then added (at the top of his voice), "you don't even remember, do you?" He was right, I didn't.

Most of us can avoid sharing air time with known racists

Well, I do now. It was in a 2003 piece about a careless toleration of antisemitic tropes among those who should know better. I cited, as an example, Ian Henshall's public suggestion that there was a link between my colleague Nick Cohen's "Jewishness" and support for a war in Iraq. And I also drew attention to his dissemination of stories designed to suggest a connection between Israel and the 9/11 attacks.

Nowhere had I called Henshall an antisemite. I am sure that, were Nuremberg-type laws to be enacted in Britain, Henshall would be against them. And perhaps, in the intervening years, he had become slightly less insouciant in his replication of anti-Jewish stereotypes.

A very brief wander round the internet, alas, proved otherwise. In 2006, Henshall has the conspiracist US Congresswoman, Cynthia McKinney, being defeated in a primary, not because she is a flake, but "after Jewish groups, accusing her of antisemitism, poured in funds and encouraged Republican voters to support her unknown Democratic rival." Henshall reiterates his suspicion that Mossad is in some way involved in 9/11.

Henshall's book, restating much of this, came to the attention of Mr Martin Webster. In 2009, Webster wrote to Henshall expressing his interest. Henshall, knowing what younger readers may not - that Martin Webster is an unrepentant and briefly famous neo-Nazi - explained why he was not keen to foster that interest. "The main attack on the 911 Truth movement is that we are 'holocaust deniers' ie antisemitic," wrote Henshall, "and it is amazing to note the key media people who have attacked us are Johann Hari, Nick Cohen, David Aaronovitch, George Monbiot, Mathew Rothschild."

He went on: "Although I am always keen to sell my book I think it would be bad tactics to be linked to your networks, because it would give these people more ammunition."

Webster obligingly published this communication on his website, where it has remained hidden until now.

You may think that some of the fruity language Henshall sent my way, could have been included in an epistle to a neo-Nazi. Clearly not. Webster is only to be discouraged for "tactical" reasons to do with what a number of journalists (including a Rothschild) may write.

After my questions but before Henshall's goodnight, Gage was asked a couple of times about the "Zionist" influences in the conspiracy, and the possibility of Israeli involvement. One woman near me, to her credit, disapproved of this line of argument.

Most didn't. Mr Gage himself has appeared on a US radio programme hosted by one Kevin Barrett, who has described the Holocaust as a "destructive myth", and alongside a copper-bottomed Holocaust denier, whose predilections were in the show's pre-recording notes.

Is Richard Gage antisemitic? No. He'd probably rather go to jail than bait a Jew. But most of us can avoid sharing air time with known racists. And just as careless is Ian Henshall. Which was the point I made about him all those years ago, and which he was still so angry about on Monday night.