Drug Abuse In The Work Place - My Employers Legal Obligations..?

I have a colleague that has been using various drugs in the work place. It has an effect on all those around him. I approached my employer in a recent one to one meeting and advised them that there is a member of staff with a serious problem. The response I received was such that they knew very well but had not caught them. This individual has a bad attitude toward every other member of staff and work. They even had their mother deliver their drugs for them on one occasion and left the drugs on their desk. This was the stick that broke the horses back for me so I handed the drugs to my superior and said "sort it out will you" Long and short is that they lost the evidence and are now asking me to give a written statement explaining all I know. Other colleagues have done this. I have now done this but with the strict request that my personal details are not divulged. They have now said that this can not be promised. Does my employer have the rite to request this statement and do they also have a rite to protect their employees. I feel like I am being made a Patsy and that they are in some way dealing with this in a cowardly and un professional way. I believe the person should have been fired on the spot. Instead they are suspended and will be having a disciplinary hearing in which our statements are now all the evidence they have.. Am I working for a bunch of monkeys here?