I have to agree the smell of weed is really strong and all so common when out walking the dog of an evening from homes cars and individuals walking past it's become so common I think the police pay very little attention to it.
However I sympathise with your predicament are you able to insulate the eves or the party wall in the loft to reduce the smell or just have a word with them next door somthing like can they smoke it in a different room or location as the smell is a problem and if that fails I would tell them that I was going to report them to the police and then do so
Before you get a knife in your back when going by your neighbours door.