Question Author
Thank you all for your very useful advice. Regarding the issue of the property boundary, the family has a walled front garden of about 20feet x 12 feet and an even larger back garden. He confines his smoking within the gardens but can be seen as the walls are only about 3 feet high.
I know that the council regard both gardens as their property as a previous tenant in the house once tried to ban a neighbour from entering the front garden of the property, and was told in no uncertain terms that he could not do so, despite the tenant threatening "trespass".
It seems to me that the guy sees himself as having some ethical and caring standards by confining his smoking to outside the property, away from the young children. I still find it deplorable and I've got little doubt he's breaching his tenancy agreement. Perhaps the first stop is indeed the local authority housing officer.