I haven't read Nancy S.'s mother's book but I have read others about the Sex Pistols and Sid Vicious. The end of Alex Cox's film is an absolute failure. Nancy did succumb to Sid's stab wounds even if the hypothesis of a third person has not been ruled out (the dealer). McLaren posted bail and Sid died of an overdose a few days later at a 'party' thrown by his own mother. At the end of the film we see them both leaving in a taxi but that's just the director's imagination.