On gangsters - have a google of the Kray brothers (the Krays, or Craze) - London brothers who ran a serious gangster racket in the East End. I'm not so sure where Sirius would come from, but if i were basing it on Gary Oldman, he'd fit in beautifully with the Kray era London - sortof grimy rebellion. London in the 70s was pretty filthy, and Sirius is just the sort to end up mixed up in it.