Image caption Callum's flat has no lockable main front door, boiler problems and no electricity in the bedroom

A union is targeting landlords across the UK, who it says are exploiting vulnerable people with poor housing. But is it encouraging people to stand up for their rights or taking the law into its own hands?

Callum Hay says his bed was so damp in the mornings, at first he thought he had wet himself while he slept.

His tiny one-bed flat in Bristol is covered in black mould.

It has no lockable main front door, boiler problems and no electricity in the bedroom - where he and his partner Zena have to use a torch.

The couple are both care workers, and have stopped paying their £510-a-month rent - including electricity and water - because, they say, they are not getting even the basics.

Their worst period was when the electricity was completely cut off because the landlord had not paid the bill.

"My partner then snapped at him, [saying] if he didn't get it put back on there's going to be hell to pay.

"He turned round and said 'I hope you die in here'," Callum tells the BBC's Victoria Derbyshire programme.

Callum's landlord - who we have chosen not to name - denies this.

He says he has been trying to do work to fix the flat but contractors have said it is too dirty to work in there.

He also claims he offered to put Callum in a bed and breakfast for a week so he can carry out work, but Callum will not agree to this.

Image caption Johnny Butcher, from Acorn, is keen to make private tenants aware of their rights

Callum contacted Acorn, a union for tenants who rent privately, which launched in the city three years ago.

With one in four families privately renting, its founders felt people had accepted poor housing and treatment for far too long, simply because they did not know their rights.

In Callum's case, Bristol City Council gave the landlord 28 days to pay his electricity bill - but an activist from Acorn went round to the landlord's house to demand he did it sooner.

"We said we would keep coming round until it was resolved. That was on a Friday, and on the Monday the electricity was turned back on," says Nick Ballard, one of the union's full-time employees.

He denies, however, the accusations of some landlords that the group are vigilantes - or are taking the law into their own hands.

"[We are] acting together in support of a particular individual or where people have a similar problem, acting together to solve it.

"It doesn't mean breaking the law. We're an entirely law-abiding organisation."

The government says councils have strong powers to ensure landlords make improvements to inadequate properties and are expected to use them.

'Power imbalance'

Acorn has around 15,000 members and supporters.

The full members pay a monthly fee - one hour's wage every month - and it operates like a workers' union, to protect any members who need help.

Its main concerns are rising rents, poor conditions and evictions.

It organises protests, goes to landlords' houses with demands and publicly shames them by telling the landlords' friends and neighbours about the conditions in which their tenants live.

This summer, 100 of its members helped barricade a vulnerable woman's house to stop her being evicted.

"We felt there was a need for a community organisation who would represent local people on political issues, but without being tied to a political party," Nick explains.

The scheme also operates in Sheffield, Manchester and Newcastle, with an aspiration to be UK-wide.

It also campaigns on wider issues, and says it has succeeded in getting Bristol City Council to scrap plans to start making the city's poorest residents pay council tax. They are currently exempt.

'Shockingly disgusting'

One woman they have helped is Sarah, who says she had unwanted sexual advances from her landlord, who lived in the same house.

Her ex-landlord has previously denied the claim.

"It was absolutely shockingly disgusting," Sarah says. "I did not want to be near him, didn't want to be in the house, I just wanted to get out. It just got worse and worse."

Media playback is unsupported on your device Media caption Sarah says her landlord made sexual advances, then tried to evict her

"I said I was completely uncomfortable with the situation and wanted him to back off and leave me alone, so he changed my six-month contract to a one-month contract. And then from that tried to instantly evict me - which is illegal."

Johnny Butcher, who works for Acorn, says Sarah was probably 12 hours away from being made homeless when they went to her landlord's house and threatened him with legal action.

"We also provided some solidarity to support her when she had to go back into her house to get her stuff - people like that are bullying landlords and we're just not going to take it any more," he says.

"These landlords are doing whatever they want. They think they are above the law.

"Sometimes people need to stand up and say, 'No, this is not right'," he explains.

The Department for Communities and Local Government says it has £2bn plans for more council and housing association homes. It is also planning reforms to the private rented sector to ensure everyone has a safe place to live.

A spokesman added: "This government is also cracking down on rogue landlords - either forcing them to improve and raise their standards or to leave the sector entirely."

Watch the Victoria Derbyshire programme on weekdays between 09:00 and 11:00 on BBC Two and the BBC News Channel.