i saw my neighbours son one night spraying a wall in london, this art , graffitti mural, i later saw in a book about banksy. The next day i asked him why he was spraying the walls, and did you know it was an offence, and that i took photografic evidence. His reply was if you tell my mum or the police he would graffity a penis and testicles on the front of my house, should i call the police??
No. But I would ask him for a couple of free originals.
Yes call the police, Not olny is it criminal damage, its also threatening a witness.
there are many many many fake banksys around these days, odds are the mural you saw wasnt banksy and i believe he lives in brighton not london.
i have been told banksy is very careful about the law finding him and i very much doubt he would threaten to draw obscene things on your house, not his style
you probably saw some kid pretending to be him, if you want to you can call crimestoppers which is anon so he wont know who told on him.