I think MI5's more open recruitment approach, on the website etc, is a PR thing (trying to reduce the public perception of James Bond secret spying) as much as it is a way of getting new employees. MI6 is supposed to be the higher level version of MI5, so perhaps you can get to work at MI6 once you've worked at MI5 for a bit. The nature of the work means it's most probably internal promotion than external recruitment. Then again it might be really easy to get a job there, it's just nobody's allowed to talk about it once they get a job, so we don't know. It could be as easy as phoning up for an interview. I'll give them a ring and see what they say....