MM - if it is not in your FCOs it should be in the c/crew SOP manual. Most EU airlines should have it somewhere, a typical wording being:
"No persons may be permitted to drink alcoholic beverage on board the aircraft unless
that beverage is served by the Cabin Crew for consumption at that time."
If you don't have it (I'm sure I had to find it once on a trip) you need to get it in place!