The job is isolating ... when people ask about it you have to be evasive

It is a very odd world to be working in: after months of recruitment you realise the sensitivity of the information you are dealing with.

It can be isolating socially because when people start asking specific questions about your job you have to be evasive. Because you cannot talk about what you're working on, I found work friends became an important part of my life. It became easy to socialise with people from the office, because I could talk shop without being concerned in the pubs around Pimlico.

That leads to a lot of office relationships, and it can get quite incestuous. I met David Shayler in the office, and most people ended up with a partner in the service.

I lived in Pimlico from 1991 to 1996, while working as a spy. A lot of officers are based in the area.

I remember someone with strong IRA connections moved into the same block of flats as an MI5 officer, and in a situation like that officers will sometimes be relocated. We were also encouraged to be careful when walking in and out of the HQ in case somebody saw you. You do have a heightened sense of security, it's always there in the back of your mind.

* Annie Machon is the author of Spies, Lies and Whistleblowers.