Not sure about figures but i was chatting to HR when they phoned to confirm my pass of stage 3, and they told me that last year they had 45,000 applicants and took 134 to college. Of these roughly 2/3 will pass college, and 3/4 will validate.
Having said that, on tower visits to LGW and LHR i was told that their failure rates on validation are much higher than other aerodromes becuase they have a harder job- around 50% fail! guess that's what you get paid more for though.
So out of 45,000 applicants last year roughly 60 people will become ATCOs- so that's about 0.1% of applicants!
3% is far too high over all- that would be 1350 a year validating as full ATCOs- and its nothing like that in reality,