Thank you all for your replies, very interesting.
500 applicants for 3 vacancies? That's tough indeed. I'm considering it as a career goal in the long term, anyway. I know that one day I will have a few chances to be at least short-listed, if the opportunity arises, but right now I've only got four years of professional experience (and I also enjoy my current job, fortunately).
Good point, M9, about private consultancy firms. I've googled RTI Forensics, for example. Securing a job there would probably pave the way for a post in the AAIB, although I suppose that the 'ethos' of such companies is quite different from the AAIB's one (I'd say that they are compensation-driven instead of knowledge- and safety-, and when the big money is involved, I start getting uncomfortable).
Deeday