One of the three men acquitted at the Old Bailey last Tuesday of trying to buy a chemical substance that police said could make a dirty bomb has described how the case has left his and his family's reputation in tatters.

Roque Fernandes, 45, of De Havilland Road, Edgware, spent 22 months in Belmarsh prison after allegedly trying to get hold of 'red mercury' with two other men, Abdurahman Kanyare, of Milling Road, Edgware, and Dominic Martins, in a sting involving the News of the World's 'fake sheikh' reporter Mazher Mahmood.

Mr Fernandes, who worked in the CCTV security room for Coutts and had previously wanted to become a police officer, said Mr Kanyare, whose son he had known from a previous job, had asked him to try to get hold of 'red mercury' for an unnamed buyer.

Mr Fernandes said he contacted Mr Martins, a former colleague whose ex-brother-in-law, referred to in court as Mr B, worked with chemicals, and might know someone who could get hold of some. Mr B then contacted the News of the World.

Mr Fernandes said Mr Martins, 45, of Du Cros Drive, Stanmore, told him 'red mercury' could be used to make explosives as well as other things.

"I was trying to stay calm and to find out what 'red mercury' was, what it was going to be used for, and who was going to buy it, to give this information to the police," he said. "But I didn't want to do anything until I got more information."

The three men were arrested on September 24 at the Brent Cross Holiday Inn hotel after Mr Martins, a Deutsche Bank employee, allegedly tried to buy the chemical from Mr Mahmood.

"I told police straight away what my role was but they didn't believe me. They said, 'why didn't you tell us?' I said, 'how could I tell you if I didn't have proper information?' Mahmood was talking about radioactive material. He was trying to get one of us to say we could get hold of it, especially Mr Kanyare. He thought all of us were going to plead guilty, but I would never have done so. Why should I if I'm not guilty?"

Categorised as a low-risk prisoner in Belmarsh, Mr Fernandes was allowed to mix with other prisoners.

"I tried to keep myself to myself," he said. "I didn't have any problems, but I was very frustrated to be there. It was a completely different world, I have never been in prison before. I have got nothing to do with terrorists, especially given the field I was working in I have 14 years' experience in security."

Despite being supported by his family and friends, he says the case has damaged his family. "My three children are traumatised because the police came to check my home," he said. My whole reputation, my family's reputation, have been damaged completely.

"I don't want to blame the police. Everybody makes mistakes, but this mistake cost me a lot. It's like starting from zero I lost my job.

"I still can't believe I'm out of prison and I still don't know what 'red mercury' is."