Of the many distressing scenes in Penny Woolcock's new documentary, Going to the Dogs, which explores the underground world of illegal dog-fighting in Birmingham, one stands out. It's of a young pitbull in training, not out in the city streets – this breed is far too demonised by both press and the police to go for anything even resembling normal walkies – but rather at home, strapped tightly by its lead on to a treadmill.
At first, the treadmill is set to slow, and the dog can walk normally. But then its owner, who, to disguise his identity, is hooded and wears a balaclava, his voice distorted through a vocoder until he sounds like a chesty Darth Vader, ups it in increments, until you are convinced that Woolcock has speeded the film up for comic effect. She hasn't. The dog is now running frantically, its legs a terrible blur. Something bad, you sense, is about to happen.
But then the machine stops, and the animal comes to a merciful standstill. It pants wildly, and as it turns to the camera, the viewer half hopes it lunges at its owner seeking retribution. Instead, it looks unaccountably hopeful and eager, as if to say: "Was that all right?"
Later, the dog will go into a crudely fashioned ring to face its opponent. Men will slap the dogs on the flank until both realise what is required of them here, at which point they will go into one another, fangs into flesh. The camera does not retreat. It lingers. At one point, blood gushes up in the dog's mouth. "Probably bitten into his tongue," someone says, breathless with excitement.
Penny Woolcock is no stranger to the difficult documentary – previous subjects include homelessness and gang culture – but a film about dog fighting, she admits, "took a great lump out of my heart. There was a period while filming when I felt completely lost, to be honest. I thought it was going to be unremittingly cruel, but the reality was somehow not quite as bad as I'd feared."
The months she spent infiltrating this hidden world were exhausting for Woolcock, now 64. It left her with nightmares, and migraines. For this reason, she has readily embraced the job she has stepped into next. You see, as well as being a film-maker, she is also a director at the English National Opera, and she is currently deep in rehearsals for a forthcoming production of Bizet's The Pearl Fishers.
Her second career in opera began late in life, when she was invited – out of the blue, the way she tells it – to direct John Adams's Doctor Atomic at the Met in New York. She went on to direct an earlier staging of The Pearl Fishers at ENO in 2010.
"It's exhausting work," she says, smiling, "but I will admit, it's a welcome change from dog fighting." She had anticipated that the dogs would be mistreated, caged, beaten before fights. This was not the case. The dogs are loved, and largely treated well. "And they were lovely animals," she says. "I wasn't scared of them at all."
The film shows how dog fighting, for many people in multicultural Birmingham and beyond, is something that has been practised for centuries. Though also illegal in Pakistan, for instance, it is still prevalent and popular there as a spectator sport.
Here in the UK, the spectators are a furtive bunch who gather in the stink and squalor of back rooms. The appeal is never fully clear: the fights are not graceful things, not balletic or strategic. Instead, they are grim, and workmanlike, the dogs fighting because they are ordered to.
The owners will tell you they fight because they love it, but Woolcock speaks to animal experts who tell a different story, that the pitbull is a particularly dutiful breed, and will do anything to please its owner. Woolcock made the documentary, she says, "because dog fighting is happening, it exists. Pretending that it doesn't won't make it go away." It was a difficult world to penetrate, but she knew people who knew people, and access was granted. Why, I ask, were they happy to talk?
"Because some people like to try to explain their world. They want to put the record straight." And their side of the story is important. "I did not want to denigrate the sport simply because it is largely run by poor people."
Class, she argues, is a key factor in our attitudes to dog fighting. And so, in order to deny the middle-class viewer the comfort of watching with righteous indignation despite having earlier tucked into a roast chicken dinner, the film also shows footage of the way in which we treat other animals: battery-farmed chickens, pigs in the abattoir, the largely privileged pursuit of pheasant shooting, and just what it looks like when a horse, no longer fit enough for Aintree, meets the barrel of a shotgun.
Horse racing, the sport of kings, is particularly brutal. More than 400 die on the track each year, and a further thousand get killed because they no longer make the grade. Why, asks the film, is not more fuss made of this?
"The dog holds a precious place in our society, privileged above all other species," she says. "So I think that there is a wider issue here: how we treat all animals." As a direct result of the film, she now eats far less meat.
"I think I'd like to do something completely different next, a romcom perhaps," she says, smiling, though this will, perhaps inevitably given her CV, be a romcom with a Woolcockian twist. "I'd like to try to bring opera and the urban-ghetto worlds together, and I'd love to do an opera with the homeless. I also want to do a film about hacking. It's getting the funding for any of these projects that's the hard part; it always is."
She is proud of the film, though readily admits she is anticipating criticism of it. Uneasy watching always prompts controversy.
"People will complain, I know, but I hope they manage to watch it all the way through, and not just switch off, disgusted, after the treadmill scene. I think what I've learnt is that very few people are born horrible. Many of them just try to do the best they can within their own context."
And, having penetrated the inner sanctum of a sport over which the law wishes it had greater control, she is half expecting a call from the police. "But I won't be able to tell them anything. I don't know a thing about who these people were, and I didn't want to. It was better if I didn't. Besides, you can't really develop a relationship with someone wearing a balaclava, can you?"
'Going to the Dogs' is on Channel 4 tomorrow at 10pm