I once had a job in which I was asked to modify the girl with a balloon. I was never given any contact details except for an email address to who I was told was Banksys manager. Considering the extent to which I butchered the artwork for a specific event I was quite shocked at how quickly I was given approval to show the piece that I had modified. I guessed that there probably was no such manager. I also guess a few people do help out with the projects but I think it's one person conceptually behind it all.