The term "Northern Soul" was coined by a soul journalist/record shop owner/label owner called Dave Godin. Before that it was just known as soul and R&B music.
The original mods had faded out by around 1970 and many had become "hard mods" a sort of forerunner of skinheads.
Lots of information out there if you Google it. Of course everyone has their own version of history - ignore everything you read by original Wigan Casino DJ Russ Winstanley, the man's only interested these days in making as much money as he can from the current interest in the music from younger people.
The film's not bad but focused too much on the drug taking of the time.
A good soul forum is Soul Source at http://www.soul-source.co.uk/
Don't have to be a member to read some of the forums. It can get a bit clique-ish but there's usually someone around who will answer your questions.
Not me though, I've only been on the soul scene since the early 90s and couldn't tell you a thing about what it was like in the 70s.
Northern soul wasn't just a 70s thing, it never went away when Wigan Casino closed down - it just went underground and venues such as the 100 Club in London, Top Of The World in Stafford and Tony's Empress Ballroom in Blackburn were still packed out every weekend. Rumour has it our very own legend Tony Morley was right into Northern Soul.