We had this, and I got so fed up I phoned the police for advice on how to deal with it.
They said first port of call was to knock and ask them to stop, in a friendly way, rather than a threatening way. If that didn't work then write a note telling them it was still bothering us and we would have to contact the police if it continued. Then if it continued, to call the police back.
So they were semi-interested.
I don't really see the point in it being illegal if the police are going to take absolutely no interest in it. Though I do get that they are WAY overstretched.
Makes it easier in our case that they are end of terrace and always smoke in the garden, so technically I guess a police officer could smell/spot them at it when "passing" (though I think they're probably, at a guess (!) too busy to drop everything and run to our street if I smell it and call them!).
Anyway, we found that speaking to them (twice) has had a pretty good effect. It's way less often now at least. Could also just be the bad weather has driven them inside...
Anyway, us mentioning that it was affecting the kids seemed to do the trick. Seems not all stoners are heartless ;)