By Chris Summers BBC News

On CCTV footage Khyam can be seen peering into a bag of fertiliser

Emma Wallis was chatting with some friends in a pub after work one day in February 2004 when one of them said: "Did you hear about that body being found in a storage depot?"

Her regional manager, Paul Best, told her he had been a witness several years before in an IRA trial.

In September 1996 the police had foiled an IRA bomb plot in London after being tipped off about 10 tons of fertiliser being kept in a lock-up in Hackney.

The mention of fertiliser suddenly made Miss Wallis very worried.

UK SELF-STORAGE FACTS There are around 600 self-storage facilities in UK, up from 100 in 1999 That is around 20 million square feet of rentable space The industry is growing at up to 15% per year The industry is worth �310m a year and employs 2,000 people About 200,000 people a year use self-storage facilities

She worked at Access Self-Storage in Hanwell, west London, and knew one of the units was rented out to an Asian man who used it to store a huge bag of what he said was fertiliser.

The bag had not moved for three months.

Mr Best asked her why someone would spend �200 a month storing fertiliser which was only worth �100.

The following morning Miss Wallis raised her concerns with her area manager, Jackie Foster, and she agreed with her.

Ms Foster contacted the police and when they arrived they were so worried they kept the place under surveillance and switched the fertiliser with an inert substance, believed to be cat litter.

Later, under cross-examination by Michael Mansfield QC, Miss Wallis said she had been aware that fertiliser was flammable but she pointed out that so were perfume, alcohol and paint, all of which the self-storage industry was happy to store.

Undercover operation

As part of Operation Crevice the anti-terrorist squad used an undercover policewoman, posing as an Access receptionist, to ring up the man who had rented the unit and query an overdue payment.

Omar Khyam visited the depot three times in March and on one occasion was caught marking the bag of fertiliser, perhaps to allow him to see if it had been moved. But by then it had already been replaced.

On the last two visits Khyam mentioned that March was the last month for which storage was required, suggesting the bomb plot was near to fruition.

On 30 March police swooped on the gang and foiled the plot.

The gang paid �200 a month to store a �100 bag of fertiliser

The bag had contained 600kg of ammonium nitrate, usually used to make fertiliser.

The gang bought it from a company of agricultural merchants and told them it was for an allotment, but David Waters QC, prosecuting, told the Old Bailey: "This was somewhat surprising as the allotment would have to be the size of four or five football pitches, and it was the wrong time of year to apply ammonium nitrate as a fertiliser."

But what are the rules for self-storage facilities and what is there to stop terrorists and other criminals from storing explosives, drugs and guns in such places?

In 1999 there were only 100 self-storage depots in the UK. Now there are about 600.

Most reputable depots ask clients to sign a form, part of which is a disclaimer in which the person waives their rights if they have stored anything flammable or illegal.

A spokesman for Access Self-Storage said they would not be commenting on Operation Crevice or the wider issue of security but a source said Access had very tight procedures and were one of the first companies to introduce a "no cash" policy.

Our members are committed to very high standards of security and insist on stringent vetting conditions for all new customers Rodney Walker Self Storage Association

But Rodney Walker, from the Self Storage Association (SSA), said: "Our members are committed to very high standards of security and insist on stringent vetting conditions for all new customers."

The SSA has 200 members, although Access is not among them.

Mr Walker said: "The process employed has been developed in connection with the relevant security authorities and comprises the collection of key data at the point of hire, such as the requirement of photo ID, the provision of full contact details and copies of utility bills.

IRA bomber Patrick Kelly's 1996 plot was foiled when police raided a storage facility

"The association also has a policy of not accepting cash payments and our members all use CCTV camera networks."

Mr Walker pointed out the SSA's rules were far more rigorous than those who rent out lock-up garages or flats.

All self-storage companies have a duty of confidentiality to their clients under the Data Protection Act.

But if there is reason to believe a storage room contains prohibited goods - such as explosives, handguns, drugs or contraband cigarettes - the company can enter a customer's rental space or open it for the police, fire service or local authority.

Mr Walker said: "We believe we have established a good working code of practices with regard to security. These practices remain constantly under review with all members providing input to help ensure that it remains difficult for individuals to use self-storage facilities for anything other than their intended use."

Worryingly Emma Wallis, and her line manager Pinderjit Saini, admitted under cross-examination at the Old Bailey trial that in November 2003 when they had hired the unit out to Khyam's gang they had had no idea fertiliser might be used to make a bomb.