The figure we were told on joining is 'In the single digit percentages'. Whether this is measured by total applicants or those that get called for an interview is uknown. Of the 14 people on my selection day, I think 5 got through. I have also heard that some days nobody makes it through. Fact: There are many more applicants than jobs to be had.
Bottom line is work very hard and walk in with the right attitude and you are likely to succeed. Worked for all I know who made it into what is, a great job.
Good luck.