Do you know where I can find some numbers on this? It's something I've always wondered. We're told by the media that 90% of heroin addicts are on the street stealing to maintain their habit - but I wouldn't be surprised if 90% of heroin addicts appear to have these "normal" lives.
Office for national statistics are nice. They're robust about what does or doesn't count, and how they count, and where you need to be cautious with the numbers. http://www.ons.gov.uk/
The other place might be Public Health Outcomes: http://www.phoutcomes.info/
EG http://fingertips.phe.org.uk/profile-group/mental-health
And finally NICE tend to have excellent referencing.
Noodle around here to find the NICE guidance. http://pathways.nice.org.uk/pathways/drug-misuse