Last Night's Television - Dispatches: Terror in Mumbai, Channel 4; The Conspiracy Files: 7/7, BBC2

Conspiracy theorists usually pride themselves on their searching intelligence. Buying the official line is for unreflective sheep, they argue. They, by contrast, will take nothing as read, hunting down the weak spots in the cover story that has been concocted to keep us with our heads down and bleating. The deep vanity that lies at the heart of all conspiracy theories is all about judgement, so it's always seemed a little odd to me that conspiracy theories also require their believers to be very gullible indeed, and to expose their gullibility in public. The Conspiracy Files: 7/7, a film about the theory that the London Tube bombings were the work of a government agency, underlined that truth with some choice examples.

I'm not talking about the kind of evidence that will always remain in dispute here. One of the theories about 7/7, for example, is that the Israeli embassy received an early warning of the attacks and that Benjamin Netanyahu cancelled his plans to attend a conference in the city. I would have thought that if Mossad had been involved in the bombings Mr Netanyahu wouldn't have cancelled anything at all. He was hardly going to be travelling by Tube, after all, and his proximity to the incidents would have added a fine artistic touch of veracity. But, setting that aside, I can see that having an Israeli secret service agent rebut the stories of early warnings isn't really going to cut it with the true believers. Of course he'd say that, they reply contemptuously. He's in on it. It's axiomatic. If it undermines the theory then it must be part of the conspiracy.

But there are pieces of evidence that the conspiracists themselves advance without apparently noticing that they make their own case risible. 7/7 Ripple Effect, the home-made documentary that is at the heart of the British "truth movement", makes much of the fact that a van from a controlled-demolition firm was pictured next to the bus that exploded in Tavistock Square. In fact, the firm in question doesn't use explosives at all, specialising in drilling and hydraulic techniques. But that's hardly the point, is it? Why, if you were planning to murder British citizens in cold blood, would you have one of your operatives drive to the job in a van advertising his lethal expertise? Then there's the issue of the four men that the conspiracists believe were framed. If explosives had been pre-planted in the Tube trains, how exactly were these innocent and unwitting scapegoats persuaded to sit in just the right seats on three separate trains at the height of the rush hour? Was Derren Brown in on it too?

It didn't surprise me very much that one of the leading 7/7 theorists, Nick Kollerstrom, has a sideline in Holocaust denial, while another – the creator of Ripple Effect – turned out to be a white-haired basket-case who believes that he's the Messiah. It would have been nice if Dr Mohammed Naseem, a senior Muslim leader who helped disseminate 7/7 Ripple Effect to his mosque, had been confronted with these facts – and had been challenged to defend the madder aspects of the theory he promulgates. But he wasn't. The result was a film that won't have successfully argued the deluded out of their beliefs and that didn't even begin on the important task of ridiculing their shameful silliness.

Conspiracies do exist of course and Dispatches' film about the Mumbai attacks showed you how lethally uncomplicated they can be. It was a grimly candid film, showing you the bloody aftermath of the killings and letting you listen in on the calls between the terrorists and their controllers back in Pakistan, a chilling dialogue between duped boys and remote-control murderers. When the gunmen hesitated to kill, the controllers urged them on, waiting on line to hear the lethal shots. And they supplied them with their PR tag lines too: "Tell them this is only the trailer... just wait till you see the rest of the film," one controller advised a gunman at the height of the attacks. "Should I write that down?" replied his nervous stooge, anxious that atrocity should follow the plan to the letter.