Where did Peter Haining take his ideas from to write about Sweeney Todd (Benjamin Barker)?

I know Peter Haining wrote a book about Sweeney Todd, not as fiction but as a biography. As I know, there are just 16 copies of this book and they're all in U.K. Unfortunately for me, I can't contact this author, 'cause he died in last November. Anyway, I hope you can help me find out where did he take that from and if there was really a demon barber... Thank you a lot!!xD