20 odd years ago...I had been doing the usual round of physics graduate interviews, GCHQ's was a little different. After getting the security pass to get in and being escorted to the interview room, they told me that I wouldn't be able to ask any questions about the job (except pay). Or rather, that I could ask if I liked, but they weren't going to answer. Weird.
The point I guess, is that GCHQ don't recruit clandestinely like spooks, even if the interview process is odd. They're part of the civil service, they advertise in the paper, and recruit graduates in the milk round.