I think the basic premise is a bit off the mark - trying to tie Northern Soul to RL is stretching it a bit, from what I can gather, soulies weren't much into RL as a rule, they have a strong undercurrent of football hooliganism attached (long thread on soul-source.com about it some time ago) but even that was secondary to the music.

Write one story or the other, but as the cliche has it... "Write what you know". Given that you know nothing about the north, northern soul or RL of that era, I'd guess you're on a loser?

The problem with sport themed films is that they appeal to the sport fans only.

The best films are as the OP implies are those that have a story of it's own, with a backdrop of the sport.
The Matt Damon film Invictus was good, but the story was appealing because of the political story that went with it. It would have been pretty shallow if it had only been a story about the world cup.
Alternatively, if its the 'buddy' movie or 'us against them' move like a Mean Machine, the sport is just a vehicle for the rest of the plot. Mean Machine would have been a good film for rugby league but of course more likely to be remade with football and Vinnie Jones.

The better use of Rugby League, that would still work now was the Tina Turner Simply the Best video. Her song quite happily stood on its own, but the video was great. You didn't have to understand the sport to enjoy the big hits or the joking faces. They made the faces more well known though. Dire Straits did a simpler thing with Walk of Life.
I always thought M People would have done something like that when Heather Small had her time. Her songs would have been perfect for an inspirational video. If I was at the Rugby League at the time, I would have paid for the video production myself, just to get it out there.

I hope the Gareth Thomas story is a good one, but it is going to be Rugby Union first. We simply need to see Rugby League as a viable alternative, rather than an end of career, Ooop North ending.

If he ever let it be written, the Ellery Hanley story would be a pretty good one. Started a long way away from where he is now.

Look up "Finn Brothers - Suffer Never" on that video site.

Wow, thanks for pointing me at that. Great footage and a great song too.
Never seen or heard of that before.

Why the connection? Are they Sts supporters?

No, idea. They are New Zealanders I think. Formerly of Crowded House.