@DannyAston - except you can't put it on your CV
One of the problems of a job in the intelligence services is that you can't put it on your CV - at best you can put some sort of generic "Government employee" title (not even sure if you can mention it was in IT).
Always wondered how that works, but of course no-one who actually knows is going to tell me :) I only know the latter as a friend did some contract work that was covered under the Official Secrets Act and was bemoaning that it was useless on his CV as he couldn't talk about the client or the work.