The film has been cleverly edited by al-Qaeda's in-house technicians and its release was trailed on Islamist websites yesterday. The footage features a message from Ayman al-Zawahiri, al-Qaeda's second-in-command, as well as film of people mixing explosives in a laboratory.

The media-savvy terror organisation has held onto this film for a year - finessing and re-editing - in order to gain the maximum propaganda impact by releasing it on the eve of the first anniversary of the bombings, which killed 52 Tube and bus passengers.

Tanweer seems to have recorded his video in Pakistan or Afghanistan on his last visit there with Mohammad Sidique Khan in the months before 7/7.

Khan also made a video which was released in the aftermath of the attacks. Both men wear the same red-and-white headgear - a scarf worn, tradition has it, by jihad warriors - and are filmed against the same backdrop. Looked at together, the films suggest a level of co-ordination, direction and leadership from outside the UK.

We know that Tanweer and Khan went to terrorist training camps in Pakistan or Afghanistan and that they met inspirational radicals in the UK and abroad. The videos add weight to the belief that they received detailed instruction in bombmaking, reconnaissance techniques and other skills of terrorist warfare.

Al-Qaeda has long prized English speaking recruits who could travel the world without hindrance on Western passports. Richard Reid, the would-be shoebomber who tried to blow up a transatlantic airliner in 2001, is a prime example. Before setting out on his suicide mission Reid, who was radicalised at the Finsbury Park mosque in London, carried out extensive scouting missions for al-Qaeda.

Men like Tanweer and Khan would have been even more valuable to Osama bin Laden, hiding out on the Pakistan-Afghanistan border. Through them he has been able to carry his war and his warped ideology to the heart of his enemy.

Al-Qaeda will have helped by giving advice, inspiration and training. In all likelihood, its leaders will have discussed with Khan a plan to target London and the transport network. But the timing and the precise targets will have been left for Khan and Tanweer to determine for themselves.

A year after the bombings, however, the biggest mass murder inquiry in British history has failed to trace anyone who helped or instructed Khan, Tanweer and their accomplices. The real masterminds behind 7/7 are beyond the reach of that investigation, hiding with Zawahiri and bin Laden.

The Suicide Factory by Sean O'Neill and Daniel McGrory is published by HarperCollins

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