Good luck if you ask for a visit; I tried last year and they were too busy to accommodate visitors, though that is probably a question of timing.
If you apply next time they advertise they send a fairly detailed info pack detailing the selection process and benefits package. They seem to advertise once every couple of years I'd say, from watching over the last ten or so. Standard thing; start at the back of flight! Mind you the Flight web site allows e-mail notification of jobs every week, which is less effort.