It is 3.15am in Hastings police station, Sunday morning. The CCTV camera on "the bridge", the raised custody sergeant's desk, is trained on cell M1. The detainee is lying on his back and pummelling the metal door with his feet. He has been doing this for more than 90 minutes, breaking off occasionally to jump to his feet and flash a Hitler salute.

Over this metallic drum roll, I am attempting to interview Kim Evans, tonight's duty solicitor whose job it is to advise those detained by Sussex police of their legal rights. Many of her clients belong to what our lord chancellor called this week the "feral underclass".

Why, I wonder, would anyone do her job? Evans has been reading Khalil Gibran, who wrote "work is love made visible". "I struggle with that at 3am," she admits. But Evans, who describes herself as "a wannabe Buddhist" and "a bit of a law geek", is made of strong stuff.

Evans spends most of her professional life in the interview rooms of this unprepossessing 1960s station that serves the seaside town. She spends two days a week and one weekend in three on call and the rest of her time defending her own clients at defence firm Goodall Barnett James. She estimates that 90% of her clients "have a personality disorder, mental health issues, and/or serious substance addiction".

Evans's day began at 9am with a call to the station to represent Chris, a 20-year-old man who has had his computer seized. He is accused of inciting a young girl under the age of 13 to engage in sexual activity. "Sometimes it can be difficult to deal with these things as the mother of a young girl but I set aside my personal feelings," Evans says.

Shopping in town with Evans is something of an ordeal for her teenage daughter, as occasionally clients come up and give mum a grateful hug. "She calls me: 'Every drug dealer's best friend'," she says. "Your Daily Mail reader might berate me for defending guilty people but if they, or their wife, were unfortunate enough to have a moment's lapse of concentration and kill a child in a traffic accident, they'd want me."

The right of people detained by the police to speak to a lawyer and have them present during an interview is presently enshrined in the Police and Criminal Evidence Act 1984 (Pace). Today, Ken Clarke has tabled a motion announcing that the UK is opting out of an EU directive guaranteeing access to a lawyer upon arrest.

Further to this, the government's new legal aid bill threatens to erode a safeguard described by appeal judges as "one of the most important and fundamental rights of a citizen" and introduced in response to a series of miscarriages of justice. If you are arrested today you are automatically entitled to free advice from a solicitor paid courtesy of the legal aid scheme. The new bill scraps the automatic right to a legally aided lawyer to advise suspects in the police station. If the proposals are introduced, suspects will face a means test before qualifying for public funds, while the bill allows for the extension of telephone, as opposed to face-to-face, advice.

Nearly 1.5 million people are arrested every year. "Many will never have been locked up before, won't know how long the police can keep them, and have no idea what to do in an interview," says Professor Ed Cape, a law professor at the University of the West of England, and a Pace expert. As the academic explains, the right to silence was effectively abolished in 1994 and a failure to tell the police relevant details that may not come up in court until months or years later can serve as evidence of guilt. "However well the police behave, police stations are worrying, even frightening, places," says Cape. "The right to legal advice is now more important then ever, but with the planned cuts to legal aid, it's under threat like never before."

Evans interviews a suspect Photograph: Andrew Hasson for the Guardian

At 5.35pm on Friday, Evans represents Kate, a vulnerable 16-year old living in a children's home with three other teenage girls. It's a fuss about nothing: a scrap among kids, and Evans's client has been charged with a very minor common assault. However, Kate has nine previous convictions, so isn't eligible for a slap on the wrist and faces a youth offending order. "Why criminalise a child for doing what kids do in her own home?" asks Evans.

Straight after, Evans introduces me to two regular clients, Jamie, Ed and their mother, Kim. Jamie, 19, can't remember how many times he has been arrested. Nearly every time both Kims have been there – his mother as "appropriate adult". It's all low level anti-social behaviour offences, often involving off-road motorbikes. Jamie is now on medication for ADHD (attention deficit hyperactivity disorder) and has since calmed down. His brother has ADHD, Tourette's syndrome and OCD (obsessive compulsive disorder).

What's it like being banged up in Hastings police station? "It isn't something that you'd ever enjoy but it gives you time to think," replies Jamie. "If you give the police a hard time they give you a hard time. If you are respectful, they'll be respectful." And for his mother? "The police talk a different language, posh words. With a condition like ADHD the police just label being "naughty". It's not seen as a reason [for their actions] even if they are on medication."

Jamie says of Evans: "I trust her absolutely 100%."

On the way to the next interview, I ask Evans whether she tries to address clients' offending behaviour? "No. I want to be the one person who doesn't lecture them." She talks of one of her young tearaways. "A colleague called him 'feral'. We used to fight over representing him in the police station. He was such a favourite, but he was like a little wild animal." He died three years ago with three other kids on a joyriding spree.

Evans is called back to station at 11.30pm on Friday in relation to a dangerous driver. Another teenager. He doesn't want a lawyer. Evans says that she advises clients to exercise their right to remain silent in about 50% of cases. "People think it's a glib and easily arrived at decision," she says. "I have half an hour to come to a decision which I live or die by and that QCs can pore over for months or years." It's also a decision that is likely to frustrate the police. Evans, who was a detective in the Metropolitan police for 24 years, says "you have to be very brave in this job. You're always on enemy territory."

If it wasn't for the relentless banging from cell M1, it would be a quiet night: only four out of 13 cells are occupied by the times the pubs finally close. Tonight heavy rain has dampened spirits. However, a stream of people are brought into the station throughout the night: mainly drunks - some incapable and looking for a kip, others kicking and screaming - and teary-eyed teens. The man making all the noise is Polish and was ejected from Yates's wine lodge for "Sieg Heiling" the bouncers. He arrives with trousers around ankles, lashing out at officers and shouting Nazi-themed obscenities. Later his wife rings up asking whether she can pick him up. He's just been inside for six months for drink driving. His parents are arriving from Poland first thing in the morning and haven't seen their son for one and a half years.

"At night there is a holding pattern," says the amiable custody sergeant Peter Hutchinson. Behind him is a poster of Statler and Waldorf, the two elderly resident grumps from the Muppets, beneath which someone has written: "Old school: apply within". "You just deal with the immediate grief: banging on the doors, urinating and defecating on the floor. People trying to self harm, covering the camera." It can be "quite repulsive", he adds.

What do the police make of defence lawyers such as Evans? "I like analogies," begins Hutchinson. "It is like a semi-detached house. You're on one side, the solicitor's on the other. You have a garden with a fence. Let's make that fence three foot, not six foot high. You don't go on to each other's territory but you talk to each other." Hutch says there is "an awful lot of trust in this place". "Unfortunately there are some custody sergeants and some solicitors that make that fence a good six foot," he adds. Evans nods. "Sometimes there are boundary disputes," she says.

Some names have been changed.

• This article was amended on 8 September 2011 because the original said Professor Ed Cape is a law professor at Bristol University. This has been corrected.