The movie was based on a comic book by Alan Moore, which he claims to have intensively researched. However, see
http://www.imdb.com/title/tt0120681/goofs
for a list of inaccuracies in the movie. The first one is "Factual Errors: There are many contradictions of historical fact (too many to list)"! One they don't mention is that the Johnny Depp character should have had a Dorset accent. Apparently the reconstructions of the murders are accurate, though. I've not read the comic book, but is supposed to be very good, and has an appendix with details of the research Moore did.