It was in an article done by the Daily Mail within the last 8 years. Since you seem very keen on finding it, I can recall it said "British Airways at Heathrow runs a course to help people overcome their fear of flying, estimated to be as many as 1 in 3."
It would be conservative to say about 30% of commercial pilots have a propensity to abuse alcohol. It's what you'd call anecdotal evidence, which I strongly believe would be nearly impossible to back up scientifically owing to the very nature of alcoholism, being a disease of denial. People are naturally going to lie about their drinking if they have a problem.