Daffyd, it's not necessarily a question of full disclosure. Now I might be wrong on the below (if I am I'm sure I'll be told!), but I'm pretty certain that the CAA have never contacted my GP. If I had a problem with say high blood pressure on my next renewal, then maybe they'd ask him for any history relating to that specific complaint, but the rest of my medical history should still be confidential.
I'm sure that there are a lot of pilots out there who may have been naughty with narcotics when they were younger, but don't do it now that they're responsible grown-ups. Maybe some will even leave posts?
PS there's a very similar thread running here:
http://www.pprune.org/forums/showthr...threadid=99671