John Stone, a salesman at Bodle Brothers agricultural merchants, was bemused by the bizarre request. The flashy young man who had arrived in a customised Audi, rap music blaring, had asked for 600kg of ammonium nitrate fertiliser for his allotment.

It was winter, thought Mr Stone, and the order was enough to cover five football pitches. “What do you want all that for?” he asked sarcastically. “Are you planning a bomb attack?”

Unwittingly, he had hit on the truth. The fertiliser was to be used by a terrorist cell of mainly British-born Muslim men to build a huge bomb. The group had discussed attacking the Ministry of Sound nightclub, Bluewater shopping centre, a train, a pub and a list of synagogues.

Anthony Garcia was chosen to buy the fertiliser because he had paler skin than the others.

Months earlier, he had been among a group learning about weapons and explosives in Pakistan. Two accomplices learnt how to prepare ricin. Another arrived with digital scales for weighing ratios of ammonium nitrate to aluminium powder. Garcia had taught the others how to dismantle and reassemble weapons.

Under the leadership of Omar Khyam, a jihadi fanatic from Crawley, they wanted to put their training into practice. But their conversations were recorded by MI5. Operation Crevice involved every officer in the south of England and uncovered dozens of suspects across three continents.

Vigilant members of the public also played their part. The fertiliser was taken to Access Storage near Heathrow. Khyam refused to answer questions about why he was paying £207 a month to store £90 worth of fertiliser. Staff contacted police in February 2004, on the same day that Khyam collected an expert in detonators from Heathrow.

The pair were heard talking about remote-controlled devices. They went to a key meeting the next day in Crawley which officers believe was also attended by Mohammed Sidique Khan, the ringleader of the July 7 bombers.

Five weeks passed before the gang were arrested. During this time they seemed “surveillance sensitive” — executing U-turns en route to meetings, using multiple names and code words, disposing of laptop computers and changing mobile phones.

Instead of sending emails, they saved draft messages and logged on with the same user names. They discussed targets, praised the Madrid train bombings and mentioned wanting to do something “sooner rather than later”.

Detectives acted when snippets of conversation convinced them that the plot was slipping beyond their control. One suspect was recorded asking whether something was “ready to go”.

On March 30, officers from five forces arrested the seven defendants and other alleged accomplices. Khyam, 25, his brother Shujah Mahmood, 20, their neighbours Jawad Akbar, 23, and Waheed Mahmood, 35, all from Crawley, Garcia, 25, from Ilford, East London and Nabeel Hussain, 20, a student from Horley, Surrey, were charged with conspiring to cause an explosion likely to endanger life. Khawaja was arrested in Canada and still awaits trial.

Explore UK News

• Crime News

• Education News

• Health News

• Science News

• Scotland News

Times Recommends

• Third of Muslim students back killings
• Curfew tames feral yobs of Cornwall
• £3,000-a-month drug free abroad