I had a similar problem recently. I live in the end house of a row of 3, and the young couple at the other end (a rental) are the same, at least the lad is. When they first moved in the air stunk, we couldn't hang washing out most days and sitting in the garden meant passive smoking.
Not just that, there's little doubt the lad is dealing. People come & go at all hours, often visiting the front or rear doors, having a quick conversation and leaving. There have also been some blazing arguments and at least once a full fight. He's nothing but a scrote - he sat smoking weed one day in the summer while his heavily pregnant girlfriend cleaned up masses of dog sh*t and her mother struggled to mow the overgrown lawn while their 2 year-old played nearby. He's not even very good at it, he coughs his guts up every time he smokes.
Also, since they moved in there have been strange noises - something like a generator seems to fire up and run for a few minutes before cutting out and will do this at all hours, that and the excessive amounts of drilling and hammering he does, for a lazy git who won't clean up his garden all seemed odd. The electric meter has also been tampered with - all apparently strong signs of cannabis cultivation.
So I got fed up with it all and reported it. A plain-clothes officer came round a few days later and agreed it was very dodgy. The middle property is owned by a single woman who feels very intimidated by the goings-on, being much closer to it than we are, she was happy to assist. He had a look but typically couldn't smell anything at the time. He then emailed a week later to say he agreed dealing was likely but couldn't be sure on production and therefore didn't feel confident the court would grant a warrant. Also said the local plod would likely carry out a visit to issue a warning, and that patrols outside the property would be more frequent.
The lad is still smoking occasionally but doesn't seem to be as often (could be that we're outdoors less now the weather has turned), and I've heard much less activity. I don't know for sure a visit has taken place but things seem to have improved.
Report it. It's all confidential, your neighbours won't know it's come from you and even if the police can't do much, a warning or door knock might make them think and cut back.
