On a Thursday afternoon at the Old Queens Head pub in north London, it's pretty quiet. There are just three or four customers at the bar downstairs, but coming through the ceiling is an insistent, Motown-like bassline. Climb the stairs and you're in another world. Skating across the parquet floor are some 200 kids spinning, dropping, sweating to a five-decades-old tune called Suspicion by the Originals. Around the edge of the floor are a few furrow-browed, older onlookers. This isn't a club, it's a practice session. It has the atmosphere of a boxing ring.

Photographer Elaine Constantine has been obsessed with Northern Soul since 1976, when she went to a youth club in her home town of Bury, Lancashire. "I saw some older kids from my school dancing in a style I'd never seen before, to records like You're Ready Now by Frankie Valli and Landslide by Tony Clarke," she says. In the year of the Wurzels' Combine Harvester and Billie Jo Spears's sappy Blanket on the Ground, this was very different from anything in the charts: "It was slightly melancholy but at the same time euphoric."

Constantine was hypnotised by the expressive dance moves of boys she recognised from school. "They seemed to be alive and on an emotional journey through each record. These were boys who to me had been quite ordinary – suddenly they were fine young things in a glorious world. Someone in there told me it was Northern Soul. It changed my life. I knew I wanted something different and wanted to belong to that special thing."

Constantine became renowned for her photographs of teenagers, first published in magazines like the Face. This is her first feature-length movie. Simply called Northern Soul, it recreates the atmosphere of soul clubs where rare records were spun, and dancers scattered talc to make moves easier. A huge underground scene, with strongholds in unfashionable northern towns like Stoke, Stafford, Blackpool and (most famously) Wigan, it developed its own codes, its own mythology. Occasionally a Northern hit crossed over into the real chart – Tami Lynn's I'm Gonna Run Away From You, or The Night by the Four Seasons – but mostly it was a private world of acrobatics, amphetamines and one-upmanship. Everyone wanted the rarest records, to be the best dancer, the best DJ. Newly discovered records were covered up with blank or even fake labels to stop other DJs finding them, too.

For Constantine, though, Northern Soul was about comradeship. "So few people understand what it's all about unless they've been involved in the scene," she says. "When I've tried to explain to people how incredible the music is and how great it is to dance to en masse, when you know everyone in the room is going through the same experience emotionally as you ... it's hard to sum up in words alone. I knew a film would give anyone the experience almost firsthand."

Though Northern Soul developed out of the mod scene, clothes were never of paramount importance; the music and the dancing were all that really mattered. Looking around the floor, as the kids restart the dance to Williams and Watson's Too Late, I see a kid in a tight white T-shirt and baggy tracksuit bottoms, and another with lank long hair who is a dead ringer for 10cc's Lol Creme. Several dancers wear one glove. One girl does the splits and springs back up – she looks about 17. Some are actors, some professional dancers, and around 20% were already Northern fans. They are coming, reckons Constantine, just because they want to have a dancefloor experience with people their own age.

"Most kids that come to the classes had no knowledge of Northern Soul. They came originally to be extras in a film, or try for main parts, and even though that may have been the initial impetus, some of them are well hooked, and they're not faking it. I'd say that nearly everyone who has been training for a few months has got into the music – it's been a fascinating experiment to watch."

Watching carefully, stopping the music if any move looks slightly out of place, is Keb Darge. Still a top soul DJ with an odd penchant for rockabilly, Darge is one of three people Constantine approached to be dance coaches; all have been on the soul scene since the mid-70s. "They encourage the kids to really listen to the music and learn each track's lyrics, breaks, beginnings and endings." The film's dedication to detail matches the intensity of the original scene: "I really don't want to have a film full of 70s-clad, rent-a-crowd extras," Constantine says firmly.

'At least I didn't break my leg'

Not everyone will make the cut: there is an air of They Shoot Horses, Don't They? (the 1969 film about the gruelling dance marathons of the 30s) to the effort some dancers are putting in. Plenty have the look, some have the brio, others have the authority, but the dancers who'll meet Constantine's exacting demands need the lot. None look too old, that's for certain. One dancer is an ice skater, and it shows; another, Danny, curls up exhausted in a foetal position as Eddie Holman's I Surrender fades away.

Actor Elliot Langridge will definitely make the movie – he plays its main character. Darge's charge, he is told exactly where to place his right hand as he drops down on to his left in a move that would probably snap several of my bones. Langridge is cute and confident. When he falls awkwardly on his hand, he takes a break. "At least it wasn't my leg," he laughs. "I wouldn't have been able to dance for a month." Does he enjoy these sessions? "Oh yeah."

Constantine says she is forever getting emails and texts from the dancers asking: what's that record? "Sometimes I go to the 100 Club and dos up north, I'm on the dancefloor, and suddenly spot one of our kids dancing opposite me! Some of them are probably more into it than I am these days." The secret sessions at the Old Queens Head must be very similar to the process that Northern fans went through when they first got into the scene – swapping moves, discovering new tunes – and it's a fair bet some of them will be on dates together after the final record, Tobi Legend's Time Will Pass You By, comes to an end.

The director herself is dancing by now – she can't help herself. And she is understandably proud of this private soul club, an unintentional by-product of her film. "The camaraderie we felt when we were young, which made us all fall in love with Northern Soul has really developed here. I reckon some of these kids have become friends for life."

Northern delight: Bob Stanley's top tunes

(At the) Discotheque: Chubby Checker

Not a typical Northern sound, being more Latin-flavoured, but this was the first rare soul single known to have changed hands for 10 quid – the price of a small house back in 1968.

Seven Days Too Long: Chuck Wood

Escaped from the cloistered soul scene as the B-side to Footsee, a cash-in top 10 hit by Wigan's Chosen Few in 1974.

I'll Hold You: Frankie & Johnny

A Glasgow duo who showed you didn't have to be from Detroit to cut a Northern classic. Frankie, aka Maggie Bell, later sang the themes to two detective shows, Hazell and Taggart.

Do I Love You: Frank Wilson

Quintessential Northern, from its joyous opening chimes; usage in a KFC ad could not dim its charms. Also the rarest Northern 45. Only two copies exist, one of which was sold for £25,000.

It'll Never Be Over For Me: Timi Yuro

This "beat ballad" was only discovered in the 80s. It may be mid-tempo, but Yuro's skyscraping performance is so intense that you've got to dance to keep from crying.

Hear these tracks, and others mentioned in the piece, on Spotify: bit.ly/pdRikk