There's a scene in tonight's Dispatches in which we see a parliamentary lobbyist persuading Norman Tebbit to lay an amendment to the Human Fertilisation and Embryology Bill. They're deep in the recesses of the House of Lords and she's telling him: "You can get a slot on the Today programme, because you can say: 'Today I'm tabling an amendment to bring down the upper limit on abortion.'"
Lord Tebbit seems persuaded and agrees to the request. Andrea (our lobbyist) wastes no time in whipping out her pre-drafted amendment and hands it to Tebbit, who dutifully takes it away with him. Job done.
As a film-maker it's one of those rare moments when you can't quite believe you're there, and that the people you're pointing the camera at are apparently completely unconcerned by your presence. The only thing to do is keep quiet and keep rolling.
I'm still not quite sure how I ended up witnessing that piece of normally strictly "off-camera" parliamentary mechanics, except that it's something to do with the nature of the relationship I developed with Andrea Williams, the public policy director of the Lawyers Christian Fellowship. Her rather dry title does her little justice. In truth, she's a colourful and powerful campaigner for the implementation of radical Christian views.
Williams says that the Human Fertilisation bill is the work of the devil, because it rejects basic biblical principles. She believes that abortion should be illegal, homosexuality is sinful and the world is just 4,000 years old. Her views on other religions, particularly Islam, are so offensive they're barely printable. When I ask if it's fair to describe her views as fundamentalist she agrees unselfconsciously.
This film about fundamentalist Christians is one of a number of documentaries I've made about extremists over the last few years. Previous subjects have included the British National Party and animal rights activists. These programmes are described as "access docs". Success depends on the willingness of your subject to cooperate and allow you to film them with an intensity that goes well beyond normal interaction with the media.
There is a temptation in television to "expose" extremist groups through infiltration and secret filming. Possibly because that sort of thing scares the pants off me, I always approach potential subjects openly, believing that, given time and by developing trust, you can get just as good an idea of how an individual thinks or a movement operates.
When a middle-aged animal rights activist was telling me on camera (over a cup of tea in her kitchen) that she'd planned to suicide bomb a laboratory, it was particularly unsettling because she was being so open about it.
One of the weaknesses of secret filming is that it encourages the viewer to understand things in very black and white terms. It records what people say or do but gives little insight into why they are saying or doing those things. Filming openly allows you to interrogate your subjects about their motivations, and this can often prove to be the really interesting stuff.
Of course, convincing people who may feel very suspicious of the media that they should cooperate with a Channel 4 documentary is not always easy. I filmed with the animal rights activists for over a year. For months I made little progress, but my break came at 4am in a muddy field. Jon Ablewhite, one of the leaders of a high-profile campaign against a guinea-pig farm, had agreed to let me follow him while he joined some other activists "monitoring" a badger-cull in Cornwall. As we waited for a pick-up, the police arrived and surrounded us. They demanded the tape from my video camera; when I refused I was arrested along with the activists. We were held for 18 hours before being released on police bail.
News that "that bloke from Channel 4" had been arrested with the highly respected Jonny Ablewhite spread quickly and suddenly all sorts of doors began to open. Another notorious activist decided to invite me on a night-time visit to a chicken farm; other veteran campaigners gave me long, frank interviews. Spending an unfeasibly long time making a single documentary is only possible because I work almost entirely alone for much of the production process. The kind of budget that would pay for a fully crewed production team for a few weeks can keep me going for months on end.
Working alone allows you to develop close relationships with your subjects. You can directly answer their questions about why you're making the programme and you can provide reassurance. Ultimately, you have to put the film together in a way that is properly fair and gives an honest account of your experiences to the viewer.
Remembering that your primary duty is to the viewer and not the contributor isn't easy and can cause uncomfortable moments. For the documentary Young, Nazi and Proud I filmed with Mark Collett, the then leader of the Young British National Party, for many months. We got on pretty well and he became increasingly keen on talking to me about his Nazi fantasies. The BNP were trying to claim that they were no longer racists and Collett was standing for election. His views directly contradicted the party's new image. In our last interview, with the camera running, I told him that I had recorded many of his pro-Nazi comments and that I intended to use them in the film. After the initial shock, and making a quick call to resign his position in the party, he continued the interview this time openly accepting that he was a Nazi sympathiser and tried to justify his views.
I had been a photojournalist for 12 years before picking up a video camera. For me, documentary-making was a natural extension of what I had been doing for years. As a photographer, to shoot people in a truly naturalistic way you have to become really unobtrusive. The objective is never to become invisible, but to get to a point where your subjects are so unconcerned by your presence that they carry on as though you're not there.
Many photojournalists use the smallest cameras possible. Big, noisy cameras are imposing and remind the subject they are being photographed. The same principles can be applied to film-making. Using the smallest video camera possible allows me to be less imposing. Even though I explain that I'm making a TV documentary, I think it's an advantage that people barely take me seriously when they look at me.
After I recorded Williams' session with Lord Tebbit, I thanked him for letting me film. He did a double take and said that at his time of life there's not much he'd be afraid of saying on camera. I'm sure he's right, but I can't help feeling that if I'd been in that room with a "proper" TV camera and a full crew we wouldn't have lasted two minutes.
'Dispatches: In God's Name' goes out at 8pm tonight on Channel 4