"What's it like to work on Newsnight?" I've been asked that question many times as I report from workplaces across Britain, but never in a place like this.

I think over the past 15 years there has been a marked change in the nature of pornography being offered. We're talking about a subtext of violence Peter Johnson, The Authority for Television On Demand

It's a small industrial unit outside Birmingham and the man in the overalls, conversing casually with me about our respective jobs, is doing a particular kind of manual labour. The kind you need to do if you are a porn actor between takes.

Outside, Laura Macrow, one of Britain's most successful adult movie directors, is getting ready to shoot a car wash scene which all concerned admit is on the traditional side of things. Man plus woman plus sponge… the rest you can do with your imagination.

This shoot will end up on the website of the actress, who charges a subscription to view it. But the big change in online porn - and what's driving the political debate - concerns the rise, over the past five years, of free sites.

Modelled on YouTube, though they of course have nothing to do with it, the free porn sites, which are mostly owned by one private Luxembourg-based company called Manwin, work to a different business model.

Peter Johnson, who runs the Authority for Television On Demand, which regulates on-demand porn in the UK, says: "The change in the business model has meant there is an enormous amount of viewing of porn around the world and particularly in the UK. On some of the free sites statistics suggest the UK accounts for 5% of global traffic flowing to the sites."

Mr Johnson believes there may be as many as 66m visits from the UK, per month, to just one of these sites.

Changing content

On sites like xHamster, also owned by Manwin, you can click straight through - with no splash screen asking your age - to porn that would have been seized and prosecuted in the 1970s, but which is now ubiquitous.

Media playback is unsupported on your device Media caption Paul Mason examines how pornography has changed both in content and availability

Mr Johnson, who was formerly head of policy at the British Board of Film Classification, says: "I think over the past 15 years there has been a marked change in the nature of pornography being offered. We're talking about a subtext of violence, hair pulling, slapping, spitting, abusive language during sex - the very extreme gender power relationships in which the man is all powerful and the woman is submissive."

And it is an issue of demand as well as supply. Once you got masses of free porn, unregulated in Britain, what happened is what always happens with e-commerce - the providers could see what the users were clicking on most, and the content changed accordingly.

"Over the years I've been involved in regulating this sector, porn producers have started off with ethos of, 'We're not going to go down abusive route we're going to make nice pornography' and have tended to move towards stronger and more abusive material," Mr Johnson says. "That's where the market drives them. That's where customers appear to want more extreme content."

In a movie if a girl asks for that, that's a consensual act which she's wanting, she's asked for it Laura Macrow, Pornography director

For Laura Macrow, who shoots for regulated TV and mainstream distributors in the United States, it has had the opposite effect. She's had to tone things down:

"We've had to tame things down slightly. We've had to change because of broadcast rules, regulation rules, we have to abide by them and follow what the regulators set us," she says.

Does she worry about the prevalence of coercive imagery in "normal" porn?

"I'm not worried about that. In a movie if a girl asks for that, that's a consensual act which she's wanting, she's asked for it. If it's given to her when she's not, then obviously we'd have to take it away in the edit. I shoot for everything on a platform and fulfil that guideline as I don't want to overstep that mark."

Meanwhile the broadband internet has changed porn in another significant way: interaction.

'Free to be different'

Victoria, a student, works as a webcam performer from her bedroom in England's Midlands. Among her specialisms is sado-masochism and her clients - men and women - pay by the minute to interact with her. She says porn plays a useful social function.

Media playback is unsupported on your device Media caption Paul Mason examines whether changing pornography content is affecting how we behave

"I think what I do, and what pornographers do, is so powerful and beautiful and positive: we help people realise it is OK if you don't want to be heterosexual. It's ok if you don't want to have the same kind of sex with the same person for the rest of your life. It's ok if you want to buy certain props or do things that are not openly discussed. It doesn't make you a bad person and it does not mean these things are bad things."

She rejects the idea that porn "objectifies" women, per se, and though she's worried about the prevalence of coercive imagery, she says it's a question of education and parental responsibility, not censorship:

Lots of young boys do have access to porn that is damaging to women - and all I have to say to that is someone is not teaching these young boys that this is not real Victoria, Webcam performer

"I don't ever claim that I help women in what I do but I think it's wrong to perpetuate a myth that it is helping men treat women badly. Lots of young boys do have access to porn that is damaging to women - and all I have to say to that is someone is not teaching these young boys that this is not real."

There are more than 3,000 women doing webcam work on one big site alone, in Britain, so it's almost impossible to even know what's in the content, let alone to police it. The fact remains that in the space of a decade, the computer screen has become a window into a wide range of sexual activity, with a greater emphasis on coercive imagery. So what's it doing to people?

Cindy Gallop set up the website Make Love Not Porn because of a change in behaviour which she attributes to porn.

"Because we have no socially acceptable language of sex in the real world, the language of porn has rushed in to fill that gap," she says. "When people are engaging in sex, men will feel an imperative to use modes of address to the partners which they hear in porn, which are not necessarily the way you would like to use to talk to someone they want to have great sex with."

Feigning pleasure

Gail Dines, professor of sociology at Wheelock College, Boston, says women report peer pressure to take part in activities, sometimes early in a relationship, that were not seen as mainstream a generation ago, but which are performed in pornography without any prior negotiation:

"It's having a profound impact. These young men who have been brought up on porn sex are expecting girlfriends to perform porn sex and these women feel often overwhelmed," she says. "When I go across the US and the UK, the women talk to me about being overwhelmed at having to perform certain types of acts, of having to pretend to like certain acts, look a certain way, moan a certain way."

What pornography is doing is giving us super normal stimuli, it's exaggerating what is a natural and instinctive desire to seek out attractive natural partners, but it is exaggerating that Paula Hall, Psychologist

She cites the changing content of porn, and its widespread availability, as the source of the behaviour change:

"Years ago when a boy, hormones raging, would find his father's Playboy he kind of got glimpses into a world of coy smiles, women bent over in a corn field. Today when he gets into the internet - via a smartphone, computer - he is catapulted into a world of sexual cruelty. There is very little soft-core porn on the internet - it has been wiped away by the industry. It is reshaping their sexuality, their sexual template and their sexual identities."

So does violent porn cause violent behaviour? It's hard to find conclusive evidence in clinical research. What is clearer is the relationship between escalating extremity of the images, and addiction.

Paula Hall is a psychologist who treats people for sex and porn addiction. She has seen demand for therapy take off.

"Some of the clients I've worked with who are addicted to porn have experienced really significant consequences as a result. Fifty per cent have lost a relationship because of it, 20% have suffered from mental health issues, 25% have sexual dysfunctions but critically about 20% have experienced a serious desire to want to commit suicide."

She is particularly worried about adolescent males. "There is more and more research suggesting porn is having a direct impact on the brain. Particularly on the adolescent brain," she says. "We know our brains thrive on novelty. What pornography is doing is giving us super normal stimuli, it's exaggerating what is a natural and instinctive desire to seek out attractive natural partners, but it is exaggerating that - the brain is becoming more wired towards those pornographic images than it is towards partnered sex."

Victoria, the webcam performer, says she can see signs of addiction among her clients, who are typically married men.

"It is addictive, it has the potential to be addictive, it's easy, it's friendly and it's warm and a lot of these clients know when they log on, I have a rapport and relationship with them. I think they forget how much money they spend and how much time they spend on me as well. Do I think it's damaging? Yes, but I don't think it's my place to tell them to stop."

Opt in plan

The government has signalled it will require internet service providers to filter out porn, unless users specifically opt in. Within the industry, views on this are mixed.

Laura Macrow thinks ISP opt-in and filtering is a good thing:

"I'm not scared by it. I shoot movies for all over the world. If people want to watch porn they should opt into it. I don't think it will have an effect on us. You will still have the people that are inquisitive these days and want to watch it. You tell someone not to do something anyway they will go ahead and try it."

Victoria, who makes a living performing via webcam, objects to the crackdown. "Porn has always found a way around this. David Cameron needs to stop trying to control people under a banner of protecting them. It's protecting nobody. It makes our jobs harder. It perpetuates the idea that what is not normal is damaging."

But the current debate on opting in or out of filters may miss the point. Never before have so many men had so much access to imagery of feigned violence, coercion and verbal abuse aimed at women, for free.

The absence of any firm evidence as to its effects so far is not necessarily reassuring. For the same language and verbal imagery is already there in the male-teen culture of computer gaming, and - as we've seen in recent high-profile trolling cases - has spilled over into the social media spectacularly.

Newsnight's full report on pornography in which we speak to experts, regulators and those who produce pornography is available for UK viewers on the BBC iPlayer.